What does 'yep' mean, exactly?

Q:

  What does 'yep' really mean?

Does it mean "Yes, thank you for telling me, i understand?"

Does it mean "I don't understand a single thing you just said, but I'm polite and you seem to like the sound of your own voice, so please keep going?"

Or does it mean "I already knew that. You've just wasted your breath talking to me. Shut up in future, big nose."

A:

  Yep

 

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