The Golden Fleece is a symbol of everything that Jason wants in the world. More than anything else, Jason longs to be a king, and he's been told that if he brings it back to Iolcus that he will be given his rightful throne. But why, we wonder, did the makers of this myth decide that it's specifically a Golden Fleece that Jason has to bring back? It's a fleece made of gold. That's kind of weird, right? Why not basket-ball shoe made of silver? Or a dump-truck made of bubble-gum? We're guessing there might be some deeper elements of symbolism to think about.
The significance of the gold is a no-brainer. Gold makes anything better, or at least we think so, as we know from the tale of King Midas. But the fleece part? That still seems kinda wack. It may be as simple as the fact that the fleece's origins are the golden ram that saved Phrixus from sacrifice. Or it may have be a symbol of masculinity, as rams boast big ol' horns that translate easily into a multitude of many interpretations. There may just be an allusion to another myth, that of Cupid and Psyche. In this myth, Aphrodite forces the girl Psyche to fetch golden wool from across a river.
A much grimmer take on the meaning of the fleece is simply that it's a totally arbitrary goal. A golden fleece is pretty much useless; and as far as Pelias, Jason, or any of the other Argonauts are concerned, there's no point in going after it expect to have something to quest after. This spells out a dark lesson about the nature of ambition: we rarely pursue things for their own sake, when what we truly desire is fame and the fate of our rivals.