The key person or thing in a group is the most important one.

A key is a specially shaped piece of metal that you place in a lock and turn in order to open or lock a door, or to start or stop the engine of a vehicle.

The keys on a computer keyboard or typewriter are the buttons that you press in order to operate it.

The keys of a piano or organ are the long narrow pieces of wood or plastic that you press in order to play it.

In music, a key is a scale of musical notes that starts on one specific note.

The key on a map or diagram or in a technical book is a list of the symbols or abbreviations used and their meanings.

The key to a desirable situation or result is the way in which it can be achieved.

If you key something in, you put information into a computer or you give the computer a particular instruction by typing the information or instruction on the keyboard.