What's a Recovery Key Id?
If you computer's disk encryption dicovered problems during boot, it may display a lock sreen, on this screen it will show you a "Recovery Key ID" the first 8 characters of this key are required to lookup your encrytion password.
Why Did I Recieve this Lock Screen?
You computer's drive is encrypted so that no one can read the contents, this protects your data from theft and misuse.
To allow you to use your data, the computer automatically decrypts your data when you boot up.
However If the system notices certain behaviors or changes to the system (like changes to the windows OS, or to the computer) the encryption system locks and request that you verify that you are authorized.