You can lock employees from tracking time on a task, e.g. if the employee has overdrawn it's budget. This is important because you must not remove employees which have planned hours on a task and already tracked time on this task. Otherwise the planned budget of this employee would be assigned to the other employees on this task which wouldn't allow you to monitor planned and spent time of this employee.
In order to lock an employee, simply check the checkbox in the 'Lock' column.
On tasks with subtasks, the lock will be inherited down to the subtasks as well. An example:
If you lock an employee on 'ProjectX', the employee will automatically be locked for 'TaskA' and 'TaskB' as well. If the employee should still be able to work on 'TaskA' but not on 'TaskB' you can remove the lock from 'TaskA' manually.