So...You could be referring to a few things, but I'm just going to run through all of them real quick.
Armature Scene Layer: The layer in which the armature is in the scene. This matters for rendering and viewport filtering.
Armature Layers: The Bone Layers in an Armature. Activating a Layer makes it visible in the editor.
Armature Protected Layers: Runs parallel to the Armature layers, activating this layer makes the corresponding Armature Layer write-protected. Users will be unable to edit bones that are in this layer.
Bone layers: Armature Layers from the perspective of bones. Determines which armature layer(s) a bone resides in.