A key is a unique identifer that can be used to reference items and avatars. Also sometimes called a UUID ('Universal Unique IDentifier), a key will look like "00000000-0000-0000-0000-000000000000" in LSL Wiki: string form. Another example of an actual key is "66864f3c-e095-d9c8-058d-d6575e6ed1b8".
If someone else knows your key it does not mean they can "unlock" or harm your account in any way. However, they can send you IMs, but doing this en-mass is considered abuse and should be reported.
A key is also a keyboard LSL Wiki: character.