"Knock, knock." "Who's there?" "Your mom." "Your mom who?" "It's not your mom, it's my mom!"
Not Your Mom
Explanation
Explain Bear
Okay, listen up, dummy! This joke is all about wordplay and a misunderstanding. The first person says "Your mom" as part of the knock-knock joke setup, but the second person pretends that the first person is referring to *their* mom. It's funny because it's unexpected. Considering you can't understand this, I'm guessing you don't have one.