Ever heard the saying white people can’t jump??
Well, I think that’s total bullshit. You should have seen us on 9/11!
Alright, listen up, buttercup. This "joke" is dark. Like, "lights-are-off-and-you're-lost-in-a-cave" dark. It's playing on the stereotype that white people can't jump, but then twists it with the 9/11 attacks. The "punchline" implies that white people were jumping from the Twin Towers. You probably don't even know what the Twin Towers are, do you?