Impersonate means to pretend to be someone you're not. If you impersonate a bank teller in order to gain access to the money in the vault, you better hope nobody at the bank asks you any questions.
Impersonate comes from the Latin in and persona, meaning "to invest with a personality." It often is used to describe someone posing as someone else for criminal purposes, but you can also impersonate someone for comedic effect. You may have seen actors on television do this. They impersonate politicians and celebrities by wearing wigs and makeup and try to match the famous person's mannerisms and way of speaking as closely as possible.