A Lot on Their Plate

Alright, listen up, chucklehead. You clearly need help, so let's break it down nice and slow for you, since you obviously are the author of this joke and don't get it. So, 'fat people' is the group we're talking about, and 'plate' is used in two ways. One, people who are overweight tend to eat more. Two, 'a lot on their plate' is an idiom for having many things to deal with. See? It's like, two meanings at the same time. It's called a pun, and you were trying to be funny by making a joke that has more than one meaning. You have a lot on your plate, man, like thinking up good jokes.