What does the hat symbolize in Waiting For Godot?

In a play without much in the way of props, the hats worn by the various characters take on greater significance. The hats are expressive of the overriding theme of existential absurdity. In the harsh, godless demimonde which the characters inhabit, there is no intrinsic meaning to anything. Whatever meaning can be attributed to objects, people, places, and situations effectively has to be invented or made up. As God is noticeable by his absence, human beings are forced back on their own resources, casting around for anything which may be endowed with significance, however ostensibly trivial.
And this is where the hat comes in. The hat symbol fits in perfectly with Beckett's exploration of the absurd, playing a leading role all of its own. A hat can bestow identity upon someone, yet at the same time, can all too easily be lost, stolen, or exchanged for another one. Identity is fluid in this meaningless world, subject-hood radically dispersed. Vladimir is forever taking off his hat and putting it back on again. He's developed a suitably absurd, almost childlike attachment to this object. But the fact that he keeps on removing it indicates that it doesn't provide him with the security he so desperately seeks. This is no security blanket, whatever he might think.
Lucky also has a fixation with his hat. He claims not to be able to think without his bowler; his lengthy monologue stops when his hat gets knocked off. Once again, the hope of achieving a stable identity proves at best temporary, at worst downright illusory. When Vladimir, Estragon, and Lucky start changing hats in Act 2 what little sense of identity there was completely collapses. No one seems to know exactly who he is, where he's come from, or where he's going to.
Yet even when the characters eventually return to wearing their own hats, their identities still remain deeply uncertain. The suggestion is that, however we try to deal with the absurdity of our existence, whatever we fixate upon to give our selves and our lives structure and meaning—whether it's hats, boots, trees, or a vaporizer—it is ultimately a hopeless task.