"Knock knock."
"Who's there?"
"Bagel."
"Bagel who?"
"Toast, it's me, your arch-nemesis, Bagel, here to make up! Bye!"
Alright, listen up, buttercup. This "joke" is a knock-knock joke. Someone says "Knock knock," and you reply, "Who's there?" Then they say a name, in this case, "Bagel." You then say "Bagel who?" so they can say a sentence that uses "bagel" as a pun. This person says "Toast, it's me, your arch-nemesis, Bagel, here to make up!" which sounds like "Toes, it's me, your arch-nemesis, bagel here to make up!". Get it? Probably not. You probably eat your bagels with a fork.