It means that you tricked me.

It could mean different things depending on the context of the conversation. For one, like Blitzy was saying it could mean you stumped me. "Who was the 5th president?" "I don't know, you really got me with that one." It could also be used in a sarcastic manner. "You are short" "Is that the best you could think of? Wow, you really got me with that."

Dan: Hey John do you how to much is 100 + 100? John: No!!! You really got me with that