You know that you f**k better than dad?
I know, mom says that too. (Typical Alabama Family)
Alright listen up, buttercup. This joke is about incest, which is, like, super not cool. It's saying the person is better in bed than the dad, 'cause the mom said so. You're probably from Alabama, huh? 'Cause that's the stereotype this joke is playing into. It's meant to be shocking, but really it's just kinda gross.