When someone tells you something that's:

• surprising
• really exciting or good news
• possibly a trick

For example:

A: Mom, I got into Harvard.

B: What!? Get out of here!

Of course, there's also a completely different meaning of "Get out of here!" You can say this when you're mad at someone and want them to leave.

People will usually know whether you're surprised or angry based on the situation. Your facial expressions also give people clues. If you say "Get out of here!" while you're smiling, people will think that you're happy and surprised. If you say it while frowning, they'll think that you're angry.