I know what that's about!
what you say when you're too lazy to listen, don't care what they're saying, or you're just plain dumb and don't know what they're talking about. It's like a conversation ender and a insult all in one.
'I know what that's about!' said Jake as he walked out of the room, clearly not listening to a word I said.
'I know what that's about!' shouted Mike when I told him I got a B on the test.
'I know what that's about!' yelled Sarah when I asked her why she was crying.