A key is a piece of metal that fits inside a lock and turns to open it. You might have a key ring with a house key, a car key, and maybe several more keys.
A key is typically cut from metal, although many hotels use plastic keys for their guest rooms. If you are told that your attendance is key, you know you have to go. Another meaning of key is a button or lever you press with your fingers, like a piano key or a typewriter key. A key is also an explanation, like the answer key for a test or the key to success. And when you sing in key, you have the correct pitch.