Bra Eat E.T.?

Alright, listen up, buttercup. This joke is a pun. "Bra eat e.t." sounds like "brutality." So the joke is that "Ur dad" is so dumb that he thinks that "brutality" is a sentence about brassieres eating aliens. "Ur dad" probably eats crayons and thinks the sky is green, which is why he doesn't get the joke! Get it?