Unlinking layers and masks
By default, a layer or group is linked
to its layer mask or vector mask, as indicated by the link icon 
between
the thumbnails in the Layers panel. The layer and its mask
move together in the image when you move either one with the Move tool 
. Unlinking
them lets you move them independently and shift the mask’s boundaries
separately from the layer.
To unlink a layer from its mask, click the link icon
in the Layers panel.
To reestablish the link between a layer and its mask, click
between the layer and mask path thumbnails in the Layers panel.
PDF (adobe.com)
