The entire play "Waiting for Godot" is an absurd illustration of human life and a portrayal, at its essence, of its futility. The characters wait for the never seen "Godot" to arrive, and while there, they encounter various other characters. At one point, the two main characters run out of carrots, and Vladimir offers Estragon a radish that he believes is a carrot which turns out to be far more bitter and less desirable than a carrot would have been.

This is reminiscent of Plato's cave in that all human experience is put through a lens of perception that we can never be certain is real. In this way, the characters don't truly know what they have, be it carrots or radishes. In addition, the characters experience and play out a typical human event—something that is supposed to be pleasant and beneficial turning out to be bitter and disappointing. This is actually a microcosm for the whole play, as the two characters wait and wait for Godot who never arrives, and the two end up contemplating suicide in the wake of him not appearing.

Your question reminds me, first off, of a famous quotation of Beckett's: "What do I know about Man's destiny? I could tell you more about radishes."

A radish, as a food, is spicy, biting, and clears your nasal passages. A radish is a product of the land and of the soil. Man is also a product of the soil from which he springs. The fact that a radish is biting and powerful might be symbolic of man's ability to be like a "radish" or it could imply Beckett's harsh and truthful message: man's plight is a bitter and acidic affair since we have to make our way in a chaotic universe....

In the play there's a point where Estagon and Vladimir are concerned with carrots and turnips. Estragon requests a carrot, is passed a vegetable, takes a bite and is surprised who angrily declares, "It's a turnip!" (14). Vladimir apologizes and "could have sworn it was a carrot" (14). This might suggest that we are never really sure of things and oftentimes the most banal and commonplace can shock or surprise us. It might be symbolic of how we "rummage" through our own lives to find things that sustain us or keep us alive. We could also quote Forrest Gump here too, "..you never know what you're gonna get..."