A gaff is a very big hook that fishermen use to catch bigger fish. It's like a really long stick with a hook at the end that the fisherman uses to grab the fish and pull it out of the water. Think of it like a really big grabber tool that you might see at a toy store or in a movie. The fisherman has to be really careful though, because they don't want to hurt the fish or damage it too much when they catch it with the gaff. It's a special tool that helps fishermen catch bigger fish and bring them back to the boat safely.