Fat is a form of body tissue on animals. It’s a squishy layer that stores energy and protects organs. Fat is also an adjective meaning chubby or thick, as in a fat piglet or a fat wallet full of money.
Some foods, like butter or avocado, have fat. Deep fried chocolate bars have a lot of fat. Humans, and other animals, need to consume some fat to maintain optimal health and absorb certain vitamins. The word fat can be applied not just to people, animals, or deep-fried food, but to any inanimate object that is notably big. Your brother likes reading comics, while you prefer fat books like War and Peace and The Complete Works of Shakespeare.