It depends on the rest of the dream, actually.
As someone else said, it could represent your future self.
It could also be a representation of how you feel, or how someone is making you feel.
It could be a representation of the fact that you are growing and learning.
It might not even be you in the mirror! It could be someone trying to give you information or advice...but on the flip side, be careful because it may be someone intending you harm as well.