A protuberance is something that sticks out, like a swelling or a lump, like a bunion on your foot. A protuberance doesn’t have to be hideous; it could be your nose on your face or a knot on a tree.
The late Latin word prōtūberāre meant "to swell," coming from the prefix pro, which means "forward," and the root word tūber, meaning "swelling." And "root word" is appropriate here, since a tuber is a thick, fleshy part of a root, like a potato. And that should help you remember how to spell the word protuberance. Don't you love it when a word's meaning and its spelling work together?