Why is it called “Peter explains the joke”?

uhh

Petahh, I don’t get it.

I really tried

Petah help I can't see it

Peter what’s wrong with the Antebellum South?

Petah! I know the kid on the left, but who is the couple on the right?

mr griffin who is this guy?

I keep seeing this in a lot of comments. I have no idea why this appears a lot. Is it a sort of inside joke in this sub?

Peetah, I need help

I don’t get it