A gaff is a long pole with a barbed hook on one end. Large fish that are too heavy to be pulled in on a line are sometimes fished with a gaff.
Think of a gaff as a simpler version of a harpoon. But rather than shooting or throwing a gaff, as is done with a harpoon, a fisherman uses a gaff as a tool to hook a huge fish and lug it onto a boat. Be careful not to make the gaffe of using gaff to mean gaffe, "an embarrassing mistake." They sound alike, but have totally different meanings!