Heidegger's "Origin of the Work of Art" is normally seen as a discussion of the nature of art, and this is certainly one of its main themes. But remember that it is centrally concerned also with the equipmental nature of equipment, with the focus being a pair of shoes. The conclusion about art (great art) is that Van Gogh's painting of shoes allows truth about the equipmental nature of the equipment to come into unconcealment. But what is that about? In order to see why this is important one must see that the equipmental nature of equipment is going to be quite a bit different for Heidegger than what we would expect. He begins his essay by discussing what it is for a thing to be a thing. He is critical of the idea that the thingly nature of thing is to be understood in terms of Aristotle's conception of imposition of form on matter, but exemplified today in the factory process of making objects for purchase. One could understand shoes in the traditional western manner as imposing form on matter to create something that serves a certain function. It could be said that the equipmental quality of equipment consists in its usefulness...but that would not be enough, we need to look at the object in its use. The problem with just looking at usefulness or the object in its functionality is that it does not take into account the meaning of the shoes as they are experienced. Heidegger's imagined example is that the shoes Van Gogh paints belong to a peasant woman. Whether or not this is true of these depicted shoes does not matter. One could tell a different story if the shoes, as some have suggested, were Van Gogh's own shoes. (He sometimes lived and dressed like a peasant, and certainly trudged in the fields as a plein aire painter.) It would be a story about the life of an artist, the lived experience of an artist. Heidegger probably chose the peasant woman because she is "close to the earth" and, in this, the existential situation, which all humans share, is made more clear than it might be if we took a modern urbanite as our example (with all the superficial overlays that might distract us). The shoes are only "what they are" when experienced as used in the fields. The point is that the shoes are experienced as connected to everything else (directly or indirectly) in the life of the wearer. For the peasant woman, this means that they are directly connected to slowly trudging in the fields, indirectly connected to the frustration when the fields refuse to nurture the crop, and more indirectly connected with other aspects of her life, including her fear of death when giving birth. This is part of what phenomenological existentialism is all about: we give an account of the structure of experience (all of the layers of experience associated with experience of the shoes) and this includes things in our lives (our existence) which are deeply important to us. Think of this as like Freud's insistence that the unconscious is always present even in slips of the tongue, and the unconscious deals with matters that are of intense emotional importance to us. So, the existential facts of life are the ones that have this deep emotional significance and they loom large in experience as a kind of intense background of which we are not entirely aware. (There is a paradox here: this background is both central to experience and also not experienced in the sense of being consciously experienced. The theory of psychoanalysis is all about this paradox.)
So how are the shoes experienced? Heidegger talks about this in terms of what he calls "reliability." There is something deceptive about this term, which means something quite a bit different from what we mean by "reliability." For Heidegger, the term refers to the way in which the shoes can have a richer, deeper significance than is found in the mere notion of their functionality. They "vibrate" in this significance. They have an aura. We do not immediately see this reliability. Rather, we see something that has a certain function. It is getting away from the functionalist approach to everyday aesthetics that reveals "reliability." Van Gogh's painting reveals the inner existential reliability of the shoes in a way that we might not get just by looking at the shoes.
It is useful to contrast this with Plato. Both Plato and Heidegger would agree that the shoes, qua objects in the realm of appearance, are not the true shoes -- that there is an inner essence to the shoes. However, for Plato, the inner essence is the way that the shoes participate in the eternal Form of shoe, whereas for Heidegger, it is to be found in the existential significance of the shoe, the way that it exhibits our human existence, that we are thrown into this world of care and anxiety. So there is something more disturbing and less reassuring in the Heideggerian vision of "essence" than in Plato's. Whereas Plato symbolizes the relationship between the Good (the highest of the Forms) and the Forms by the light of the sun, which he also calls "truth," so too Heidegger symbolizes the presence of "reliability" by the way in which something seems to shine or have an aura. The shining of the shoes in experience shows that their existential essence is shining forth, which is the same as saying that their truth is shining, and Heidegger calls truth "unconcealment." Another feature of the shoes that Heidegger observes is that they have a certain life in experience: the usefulness can gradually disappear (partly because of old age and brokenness) and they can become merely usual. This refers to a major aspect of everyday life, when everyday life is boring. It is through art that we can be reintroduced to the unconcealment of Being in the world of everyday objects and in which the entropy of the boredom of the everyday is overcome.
There is a disagreement between myself and some Heidegger-influenced authors interested in everyday aesthetics. For instance, Arto Haapala believes that one ought to focus on the ordinariness of everyday aesthetic experience and set aside that which is strange. As I see it, the approach I have taken is more in tune with Heidegger which, of course, does not mean that it is better because of that (one would not want to be "in tune with" Heidegger's Nazi beliefs evident at the time he wrote this essay!) The essence of the everyday aesthetic experience is when truth shines forth, when we get "reliability" and not merely boring usualness, when we manage to break beyond the western emphasis on the notion of informing form on matter, when we actually break down the form/matter dichotomy so that the thingly nature of the thing comes forth as something essential to it and not something that merely is imposed upon by its formal nature. The point, however, in favor of Haapala's interpretation is when Heidegger says that only when the peasant woman wears the shoes in the field are they what they are and "they are all the more genuinely so, the less the peasant woman thinks about the shoes while she is at work, or looks at them at all, or is even aware of them." But I do not see how this can be made consistent with the idea that "in the shoes vibrates the silent call of the earth." There must be some experiential dimension to the shoes that is understood in the shining forth of truth that is so important to Heidegger. Heidegger himself helps us when he says "if only this simple wearing [of the shoes by the peasant woman] were so simple": she knows about the complex existential laying of significance "without noticing or reflecting." So the usefulness of the shoes is not what we call usefulness, but rather "rests in the abundance of an essential being of the equipment" which he calls "reliability." It is by this reliability that she can know "the silent call of the earth." It is by this that she can also be "sure of her world" by which is meant the world of a peasant woman.
For me, a central neglected passage is when Heidegger says that usefulness depends on reliability, which is to say that the primarily thing is the dynamic way in which the shoes fit into an entire way of life. You would not have usefulness if you did not have things that underlie it. This is what is meant by "the former vibrates in the other." So, what about the large portion of things in our world that are tired and boring? Heidegger covers this when he says, "A single piece of equipment is worn out and used up; but at the same time the use itself [the reliability] also falls into disuse, wears away, and becomes usual. Thus equipmentality wastes away, sinks into mere stuff. In such wasting, reliability vanishes. This dwindling, however, to which use-things owe their boringly obtrusive usualness, is only one more testimony to the original nature of equipmental being." Heidegger's philosophy is directed not only to the western forgetting of Being (to be found in the production of objects for mere use, and their reification as fetish objects in a world of mass consumption and death of spirit) but also in the "boringly obtrusive usualness" of the the loss of the shining aura of Being which is found in the ordinariness of the ordinary. We tend to think of everyday objects in terms of "worn-out usualness": we tend to think of this as "their sole mode of being." It is only through great art and through seeing the world as an artist would that we can recover this other mode of being.