When do people say "that's what I'm talking about?" I heard it quite a few times.

Native speaker’s answer
Rebecca
The phrase "that's what I'm talking about" expresses enthusiastic support for whatever "that" is in the context! It's similar to "that's excellent" or "that's great!" Ex: That's what I'm talking about! Our team deserved that win. Ex: A: Okay, you've convinced me. I'll come to the party. B: That's what I'm talking about! I'll see you later.