If a user enters the wrong password when trying to login, the account will be locked, and a manager will have to unlock the account. A user account can also be locked manually if the user is no longer allowed to login.
If an account is locked, it will display in the People list with a padlock symbol next to the user's name:
To unlock the account, click on the user and go to the "Status" section in People details. There you'll see that the checkbox for locked is checked. Uncheck it, and save the People details page:
The padlock icon will be removed, and the user is now able to login again:
To manually lock an account, just tick the checkbox for locked and save the People details page. The padlock icon will appear, and the user can no longer login.