If what you've seen is the simple word "Bump," it means that someone typed that word as a response to bring the whole thread back up to the top because they think it's important enough that everybody... or more people... read it.

If you see a post by "Hillbunny" that says "Bumpies," it's because she has a bad reaction from {{{{hugs}}}}. She'd rather "bump" and be "bumped." She's really a sweety! hehehe