The
command is a toggle that switches from editing the layer, to editing the layer mask (if present) and vice versa. When the layer mask is in editing mode, a white border is shown around the mask in the Layers Dialog.Note that editing a layer mask is separate from showing the mask. You can make the layer mask visible to make it easier to see what you are changing on the mask.
You can access this command
From the main menu through
→ → .From the Layers Dialog, you can access it through the command of its context menu; or you can click on the thumbnail of the layer mask to make it active.