In some cases you might have multiple team members working on a media schedule at once. Which means updates and changes can happen fast without you knowing. If there are items that you don't want to be changed while you are working on them or waiting on client approval there is the option to lock them. This prevents them from being edited or even deleted from the plan.
To lock an item all you need to do is select the placement or package and then from the action bar click on the lock icon. Two options will appear - lock and unlock. Once an item is locked, it can't be edited or removed from the schedule - only you or an administrator can unlock for further changes.