But how does all of this sum up, students often ask. Here goes an attempt. (Whatever I will say here will probably seem like an oversimplification or distortion to someone.) For Heidegger the essence of art (great art is all he is interested in) is that it reveals truth. But truth is not scientific or science-like in this case: it is Being coming into unconcealment. But what is "Being"? It is the inner meaning of things. It is what we are no longer good at listening to. It can be seen as the inner essence of things, if we understand essence in a much more dynamic and historically embedded way than allowed by Plato and Aristotle. Heidegger is an atheist, but the most religious atheist I know. In this regard he is somewhat like a Zen Buddhist who tries to get us to attend to things without presuppositions.
In order to understand what art is we need to understand that it exists and thrives in an dynamic relation between artist, art object, and art (as in, the artworld) itself. Heidegger's approach to understanding art, as with everything else, is both phenomenological and existential. The phenomenological part is that he attempts to understand the essence (or, rather, the origin, the dynamic emerging) of something by way of a certain kind of observation and consequent description (phenomenological description.) By doing this he interestingly arrives at a similar place to Dewey's. Both philosophers believe that one should start with experience, and both understand experience as something that is rich and deep, not as a mere collection of data. To understand art, we have to start with our experience of art, but also with the experience of the artist, since it is the artist who experiences the creative process and gives rise to the product which we then experience.
So Heidegger looks at the question "what is art?" and asks whether this can't be understood in a simple way, that each art work is a thing, and art is the collection of all of these things. But this does not capture the dynamic nature of art. He then considers what a thing is.
This is where his philosophical discussion of art gets deep. There is a traditional notion of a thing which understands things as if they were pieces of equipment, for example a hammer or a shoe. A traditional view of things is that they are just examples of form applied to matter. But if you just attend to the thingly nature of a thing this idea begins to dissolve. That is, if you look at Chartres Cathedral and really see that it is made out of stone you also see that this stone is not mere stone: it itself vibrates with meaning. You become more aware, in the case of art, of the medium. But even afterwards, when looking at stones in nature, you become more aware of the way in which the stones themselves can vibrate with meaning. Back to the work. You notice the stonely nature of a work of architecture made out of stone.
So there is a view of art which says that it is just a physical thing with an added symbolic dimension, the symbolic dimension being the real meaning of the work. Collingwood seems to have had a view like that. You could say that Danto held this view as well. So it is a prominent view even today. Heidegger rejects it not because he thinks that the work is all the material thing or that he thinks it is all the meaning symbolized: rather he seeks to deconstruct or dissolve the very notion of thing as matter shaped by form, i.e. where the two are radically distinct.
So he uses the example of Van Gogh's painting of shoes to show that the thingly nature of a thing is not at all just the material side of a thing. In describing Van Gogh's painting Heidegger seems at first to be engaging in a sort of wool-gathering, as though he were imagining a particularly tough life of a peasant woman who may have worn these shoes. What he is actually doing is recognizing (much as Schopenhauer does when talking about the capacities of the artistic genius when observation a physical thing) that the artist is able to see deeply, in an existentialist sense, into the piece of equipment he is depicting, i.e. the shoes. By the artist looking closely at the equipment he sees it in the context of its world, the world of the peasant woman who uses them. And by us looking closely that a painting of this we too can see this world.
Heidegger does not mention this, but we also see ourselves into this world, as we share certain fundamental existential realities with the peasant woman: we are all beings heading to our deaths, and this is part of the way we experience the world. You cannot see a painting of worn shoes except by thinking of them as things worn hard by a person who exists in her own phenomenological space. The point is that the shoes do not symbolize something other than themselves. Rather this is their inner being, i.e. as present in the world of the peasant woman, i.e. as a window to that world. Nor does the painting symbolize something other than itself: it is in seeing the thingly nature of the painting that we see Being, that Being comes into unconcealment.
So what is the relevant of all of this to us in the 21st. century? We are surrounded by things but we do not notice the thingly nature of these things, nor do we notice the equipmental nature of equipment, i.e. what Heidegger calls reliability. We are surrounded by things that have for us, usually, a kind of boring usualness. Life, as a result, seems meaningless. We the purchase more things to fill in the gap. All we get is formed matter, matter that itself is inanimate and which has its meaning only by having form imposed on it. Primitive peoples perhaps do not experience the world in this way, but we postmoderns are alienated from things, as we are from people. For we also treat people as things; not as things ought to be treated, but as mere things. Just as a work of art is not a physical object with a meaning appended so too a person is not a physical body with a soul appended. This view of artworks and persons distorts both the physical side and the spiritual side. Again, these two sides need to be dissolved together.
When Being shines forth in a thing then its Thingly nature shines forth, and this means that what Heidegger calls "reliability" shines forth. The funny thing about this word "reliability" is that it means something quite different from what you would think it means: the reliability that shines forth is the essentiality of the thing, or better the Being in it coming to unconcealment. This is why it shimmers, it shimmers with Being. Art then gives life meaning by bringing us back to the point where we can see that the equipmental natura of equipment, the thingly nature of a thing, and the workly nature of a work, are essentially the same. You cannot reduce the work to the thing when the thing is taken as a mere thing, but then if you see the thing as something that manifests a world while at the same time doing through through its materials, through its medium, then Being comes forth.
So that's the theory in a nutshell.