to do an impression of someone is to act like someone else.
take for example, Robin Williams. He does impressions of other people.
For example, lets make up a friend. We'll call him Fred. He is into math, chess, and quadratic equations. lets say you on the other hand, are into sports, country music, and can never understand a word he says.
If you were to do an impression of him you would change your voice to sound like him and say something like "Look at me, I'm Fred. 2+2 is 4 and you can't move your knight to E5 because that is physically impossible"