A user gets locked out of their account after they fail 5 login attempts. This is a security measure to prevent others (with possible malicious intent) from gaining access to your account.
However, sometimes the user didn't remember their password and tried logging in a few too many times. If a user gets locked out of their account, an admin can unlock it for them again.

How to unlock a user

Go to the user overview by clicking Users > Users.
In case a user is locked out of their account, you will see Unlock in the options on the right.

Simply click Unlock, and they will be able to login again.