When 掉 is used as a verb complement (not a verb), it indicates that something disappears, is removed, is disposed of, etc. as the result of an action. Not really "falling" or "missing".
擦不掉：Something can't be removed or got rid of by rubbing or wiping. Maybe it's a stain on your shoes, and you're trying to rub it off with a cloth, but it can't be wiped away.
吃掉：To "eat something up". All of it is gone.
卖掉房子：To sell your house. After you sell it, it's gone. It's off your hands now.
忘掉：To forget something. After you forget it, it's gone from your memory.
忘不掉：To not be able to forget something, to be unforgettable.