Hook can mean several different things:

1. A curved piece of metal or plastic used to catch, hold, or hang something. You hang your coat on a hook by the door, or a fisherman uses a hook at the end of a fishing line to catch fish. The curved shape lets the hook grab onto things and hold them securely.
2. To catch or fasten something using a hook. A fish gets hooked when it bites the bait. You might hook your backpack strap over a chair, or hook your fingers around a rope to climb.
3. Something that grabs your attention and makes you want more. A great book has a hook in its first chapter that makes you keep reading. A catchy song often has a memorable hook, a part that gets stuck in your head. Teachers sometimes start lessons with a hook to get students interested right away.
4. In basketball, a hook shot is when a player shoots by sweeping the ball up and over their head in an arc, using one hand. The shooting motion looks a bit like the curve of a hook.

When someone says they're hooked on something, they mean they can't stop doing it because they enjoy it so much, like being hooked on a new video game or hooked on reading mystery novels.