Laying one picture over another, with a single slider covering both directions. Push it one way and the first picture sits on top; push it the other and the second does. In the middle, neither is applied and you see the pipeline image untouched.
What decides where the top picture is see-through is its transparency, which means it only works on something that already carries transparency with it.
Mask mode drops that requirement. Instead of reading the second picture's transparency it reads its brightness, and uses that as the transparency of the first. Now anything at all can act as a stencil: bright areas of the mask keep the picture solid, dark areas cut holes in it.
That is the difference between laying down something that already knows its own shape and cutting a shape out of something that does not.