A stooge is magician's confederate who pretends to be a spectator. A stooge may fake amazement, deny his previous personal contact with the magician, behave in a certain way, say certain things, lie about something...
The main purpose of a stooge is to make a trick look less possible than it really is. He may pretend that the magician has read his mind, made him levitate or cut him in half. He may confirm magician's statements which are untrue (e.g. The box is empty, We havent met before...).
Many magicians disagree with the use of stooges. Others continue to use them in their acts. It is a question of personal views on magic and what is acceptable in it.