Cheesy Pizza

Listen up, buttercup. The joke is that the person was going to tell a joke about pizza, but then didn't because it would be "too cheesy". "Cheesy" is a slang term that means corny, sappy, or low quality. They are also making a pun using the literal cheese on a pizza and the word "cheesy". See, I have to spell it all out for you because you are dumb.