Tumbleweed is the first thing we see in the movie. As it rolls across the desert plains, we hear the cowboy's voiceover setting the scene: telling us (in not-too-specific terms) where we are and what the story is going to be about. Then the tumbleweed rolls into places we don't usually associate with tumbleweeds: a beach, Hollywood Boulevard, etc.

The tumbleweed, if you haven't guessed already, is symbolic of The Dude. Like The Dude, it's a relic of a different era—the wild, wild West in this case—that's a little out of place in its current surroundings, like the cowboy at the bar. And also like The Dude, the tumbleweed doesn't really seem to mind that it's out of place: it just keeps right on rolling. The tumbleweed helps us establish the visual narrative of an eccentric-but-ambitious loner making his way in a hostile world, blowin' in the wind and wandering aimlessly, just drifting wherever life takes him.