What does the layer mask checkbox do?

[Mr. K]

I'm experimenting with layers masks. I cannot figure out what the checkbox to the right does. I click it and nothing happens. I can show or hide the mask using the Quick Mask button (Q) in the top tool bar.

[R C-R]

That checkmark controls layer visibility, so in effect it turns on & off the mask. If the mask is all white nothing will be masked so there will be no difference in how it affects the layer it masks whether it is on or off.

So try selecting the mask layer, selecting a paint brush & a dark color like black & paint a bit on the mask. You should see the difference as you toggle on & off the mask layer.