The command merges the selected layer masks with the selected layers. The transparency information in the layer masks is transferred to the alpha channels of their respective layers. If a layer doesn't have an alpha channel, it is created. When that is done, the layer masks are removed.
If the selected layers do not have layer masks, this menu entry is disabled.
See the Layer Masks section for more information.
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 by
          Shift+Ctrl
          clicking on the
          
            
          
          icon button on the bottom of that dialog.