The Bifrost Bridge is straight out of mythology: a rainbow that the Norse Gods use to conveniently get back and forth to the mortal world. Here's it's pretty much the same thing: an easy way to get Thor and his buddies from Point A to Point B without thinking about it too much.

The symbolism comes in its connection between Asgard and Earth, and what happens when that connection is cut off. For most of the film, Thor doesn't get to use the Bifrost, since he's in big trouble with his dad: reminding us that "exile" is for real in this universe.

That comes full-circle at the end, when Thor pretty much beats up the Bifrost in order to stop Loki for killing all the frost giants. Early on, it's a reminder that he's done wrong. At the end, though, he chooses to sever that connection for a greater good, even though he knows he's be cut off from someone he cares about again.

First he's stuck without it, and then he chooses to destroy it because it's the right thing to do. That makes it a good symbol for what he's learned during the movie, and how his actions help define him as a hero.