You can lock layers to protect it from accidental changes to its position and scale, and to prevent the layer from getting in the way while selecting other layers in the scene editor. This is particularly helpful if you are working with Green Screen or Chroma Keyed layers and need to interact with other layers in your scene. 

How to lock a layer

1. Right-click on the layer in the scene editor.
2. Select Lock layer. You will no longer be able to select, move, or scale this layer in the scene editor.
   

When you now view the Layer panel, your locked layer with have the lock icon next to its name in the Layer section.

Locking extended layers

Layer locking occurs on a per-scene basis. If you have a layer extending across multiple scenes, you'll need to select the entire layer from your Timeline:

1. Select your entire extended layer by double-clicking it in the Timeline. You may need to enable show all layers first.
2. Lock the layer — you can use Option + L (Mac) or Alt + L (Windows).

Unlocking layers

You won't be able to select a locked layer from the scene editor, so you'll need to unlock it from the Properties panel.

1. Select the scene of your locked layer — click anywhere outside your scene in the scene editor, or select the scene thumbnail from the scene rail.
2. Click the lock icon to unlock your layer.