Adds a layer mask to the currently active layer. A layer mask controls the transparency of the current layer . The mask itself is a grayscale image where black pixels are transparent and white pixels are opaque. This works on a grey level also, allowing gradiented masking.
Adding a mask to a layer has the same effect as adding a second Alpha Channel for that layer. In fact, both are applied to the layer in the same way, first Alpha, then Mask.
The Add Mask Options dialog allows you to choose the initial fill of the layer mask. Black will make the layer totally transparent; White will make the layer totally opaque, or you can choose to initialize the mask using the alpha channel of the layer. When you choose the alpha channel the transparency of each pixel will be mapped to a grayscale value in the mask. If you want to further manipulate the transparency in a layer, this option can be useful.
To paint on the layer mask you have to activate it. You do that by clicking on the thumbnail image of the mask. However you will not see the mask, you will just see the result of your mask applied to the layer. To see the mask you have to Alt-click on the thumbnail. If you want to see you layer without the mask applied, Ctrl-click on the mask thumbnail. Remember to click once again with the same modifier key to exit the current "preview" state.