What does “you're joking” mean?

used to show that you are very surprised at what sb has just said

How is “you're joking” used?

In American History X (1998), one line is: “Oh, God, man. Please tell me you're joking.”

The clip at the top of this page is from American History X (1998).

Oh, God, man. Please tell me you're joking.

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