What does the term '20 questions' mean?

It is a game - you have 20 questions that you may ask another person to find out what he or she is thinking about. Most people start with the question "Is it animal, vegetable, or mineral?" to narrow things down.

You can also hear this used in the phrase "We're not playing 20 questions here," meaning that somebody is asking too many questions and the other person is not going to answer them.