What Does The Phrase "To Burn Your Bridges" Mean?

This can also be "to burn your boats." it is the same in each case; it means that you make it impossible to go back. The term is probably military in origin, but it is used as a metaphor for any decision that you can't take back.
An example of burning your bridges: you want to change jobs, but instead of waiting until you have found something new, you hand in your resignation first. Now even if you don't find a new job, you can't stay in the old one.

This doesn't mean that "burning your bridges" is always considered a bad idea. There are situations where the only thing is to make a decisive move, so that you have to stop hesitating and deal with the situation. For instance, it may be better just to walk away from a destructive relationship than to keep waiting until you've sorted out exactly what you're going to do next.