Locking a mask prevents you from selecting it in the Timeline, Composition, and Layer windows or setting it as a target in the Layer window. Use this feature to avoid unwanted changes to the mask.
To lock or unlock a mask:
In the Timeline window, expand the Mask property.
In the Switches column, click the box underneath the lock icon () next to the mask you want to lock or unlock. A layer is locked when the lock icon appears in the box.