Masks and matte

Could someone give me a simple explanation on how I could take a black and white image/ Video, i.e., a solid white cube on a black background, and turn it into a mask so that it edits the alpha layer of another clip? A node setup would work fine.


I meant in the node editor. I want to make it so that the white area shows one clip and the black area shows the other.

Oh! I see! Thanx!