The common hook is a J shape with a small barb at the point of the hook.
The top of the hook is the top of the J. It has a small hole where you tie your line to the hook.
The bottom of the J is where the hook point is. When the fish bites the hook, and swims away - the hook goes to pull out of the fish, (since it’s tied to your line ) and gets caught in the lip (because the mouth is closed ) The fish is now hooked on. The barb keeps the hook from easily sliding out, because of its design it pins the hook into the fishes lip.
Lots of bass fisherman set the hook ( meaning when they feel a bite of a fish they pull up or to the side quickly, pulling the hook hard into the lip of the fish. Meaning it has a better chance of staying on!