9 + 10 = 21

Listen up, buttercup. The joke is that the person answering "What's 9 + 10?" with "21" is supposed to be stupid, like you. "My name Jeff" is from a Vine, so you're probably too old and out of touch to get it. Ligma is a made-up disease, and people are tricked into asking "What's ligma?" so the joker can respond with "Ligma balls!" It's basically the peak of comedy for folks with the IQ of a toaster. You're welcome for explaining the joke.