Layer and vector masks are nondestructive, which means you can go back and re‑edit the masks later without losing the pixels they hide. In the Layers panel, both the layer and vector masks appear as an additional thumbnail to the right of the layer thumbnail. For the layer mask, this thumbnail represents the grayscale channel that is created when you add the layer mask. The vector mask thumbnail represents a path that clips out the contents of the layer.
To create a layer or vector mask over the Background layer, first convert it to a regular layer (Layer > New > Layer from Background).
