Lock a layer
To lock a layer may be useful under several circumstances. Click under the little padlock located in the Timeline in front of the layer to be locked.
Once a layer is locked, it is no longer possible to draw on it. You will notice that the look of the layer in the Timeline has changed : there are no longer strips to increase or shrink the number of instances, the head and tails of instances are no longer visible, no more pre or post behavior icons, and of couse, you can't change its position.
Click again on the padlock to unlock the layer.
By clicking on the main padlock in the Timeline, you will have access to several options that will allow you to act on several layers at once.