The Show Layer Mask command lets you see the layer mask better by turning the image invisible. When you click on the menu entry, a check is displayed next to it and the layer mask's thumbnail in the Layers Dialog is shown with a green border. The layer itself is not modified; you can turn it visible again later.
You can access this command from the image menubar through Layer → Mask → Show Layer Mask,
or by holding down the Alt key (Ctrl+Alt on some systems) and single-clicking on the layer mask's thumbnail in the Layers Dialog.
You can undo this action by unchecking the menu entry in the Layer → Mask submenu or by Alt-clicking (or Ctrl+Alt-clicking) again on the layer mask's thumbnail.