When you click on the Edit Layer Mask item on the Layer Mask submenu, a check is displayed next to it, the layer mask becomes the active component of the current layer and the layer mask is displayed in the Layers Dialog with a white border. When you uncheck it, the layer itself becomes the active component and it is displayed with a white border. You can also activate the component you want more simply by clicking on it in the Layers Dialog.
You can access this command from the image menubar through Layer → Mask → Edit Layer Mask.
You can undo this action by unchecking the menu entry in the Layer → Mask menu or by clicking on the layer component in the Layers Dialog.