Locking is useful for preventing accidental changes to a layer. A locked layer displays a crossed-out pencil icon in the Layers panel. Objects on locked layers cannot be selected or edited directly; however, if objects on locked layers have attributes that can be edited indirectly, they will change. For example, if you edit a tint swatch, objects on locked layers using that tint swatch will reflect the change. Similarly, putting a series of threaded text frames on both locked and unlocked layers will not prevent text on locked layers from recomposing.
To lock or unlock one layer at a time, in the Layers panel, click a square in the second column from the left to show (lock) or hide (unlock) the crossed-out-pencil icon for a layer.
To lock all layers except the target layer, choose Lock Others in the Layers panel menu.
To unlock all layers, choose Unlock All Layers in the Layers panel menu.