For some people, when we're very scared, shocked, or offended, our faces suddenly become pale. That is, in response to a nasty surprise, our faces look whiter, possibly because we're suddenly getting less blood flow there. This paleness, or pallor, is the idea behind the word "appall."
When something appalls you, it scares you, shocks you, disgusts you, or somehow freaks you out, and it's as if your face is losing its healthy color because you're so bothered and surprised.
When something is entirely unexpected, and just awful, you could say, "That is appalling." Recently I saw a child crush a worm, on purpose, for no reason. We were outdoors. The worm was minding its own business. The child was plenty old enough to know better. I was appalled. I found it appalling that someone would end a little creature's life that way.
Things appall us when they're nasty, unfair, or cruel. You'd be appalled by someone openly sneezing onto a birthday cake. You'd be appalled by a cockroach crawling over your toothbrush. And you'd be appalled by a big kid shoving a little kid to the ground.