What does it mean when someone calls you a mole?

A mole, a rat, a snitch, a stool pigeon - basically, an informant, someone who will divulge privileged and often incriminating information acquired from a circle of persons that deem him/her to be trustworthy to some form of competing party, in exchange for some advantages.

As a simple example, the Police have moles (informants) infiltrated in the Mafia, but we may also call moles those Police officers that are corrupt (i.e. on the Mafia's payroll).

Another simple term for a mole would be "traitor".