Yo mama is so fat when she goes to the dentist, they make her lay face down.
Yo Mama at the Dentist
Explanation
Explain Bear
Listen up, buttercup. This "yo mama" joke is saying your mom is so HUGE that the dentist has to have her lie on her stomach instead of on her back in the chair. Why? Because she's too wide to fit or would break the chair. You probably wouldn't understand, you small fry.