A simulacrum is a fake version of something real. A wax museum is full of simulacrums of famous people.
Simulacrum comes from the Latin word simulare meaning "to make like" and is related to words like simulate (to imitate) and similarity. A simulacrum might look like a person, but it's usually a sculpture. Also, a simulacrum can be a representation that's not very good. If you say, "This video game is only a simulacrum of playing football!" that means it does a poor job of copying the game.