Masking an image is like poking a hole in the background to reveal the foreground. I use this analogy because A) I’m growing a bit weary of the “white reveals, black conceals” line; B) I’m just coming off a ski vacation in Big Sky, Montana, at the end of which I wiped out on a bunch of rocks at the top of the 11,166-foot Lone Peak (see diagram below) and poked a purple-looking hole in my knee; and C) masking is exactly like that, except not so purple.
What does that have to do with alpha channels? The modest but powerful alpha channel serves as a sort of cozy base camp for the hole-poking mask. That is to say, a single alpha channel lets you create a mask, refine that mask, perfect it, and save it for later use. Watch the video below to see what I mean.
If the above video doesn’t play for you, you can watch it on YouTube.