You can fully or partially lock layers to protect their contents. When a layer is locked, a lock icon appears to the right of the layer name, and the layer cannot be deleted. Except for the Background layer, you can move locked layers to different locations in the stacking order of the Layers palette.
Click the Lock All icon at
the top of the Layers palette to lock all layer properties. Click
the icon again to unlock them.
Click the Lock Transparency icon at
the top of the Layers palette to lock the transparent areas of the
layer, so that no painting occurs in them. Click the icon again
to unlock.