Impersonation is when someone pretends to be another person. If you pretend to be your twin brother all day at school, that's impersonation.
There are a few different forms of impersonation — some of them are harmless, like comedians who use impersonation to mimic the voices and mannerisms of famous people, or actors who play historical figures in movies. Other kinds of impersonation are harmful, including when a thief takes someone's identity (including Social Security number and bank information) in order to steal their money. Impersonation has the Latin roots in-, "into," and persona, "person."