"BUMP" is to refresh the post? I don't know if I can word this correctly. "BUMP" means to ,,bump up the post into relevancy on the 'New Post' page so that more people will see the post. At least that's what I think.
To bump the post means to put it as the top post or the recently replied post. It's for people who wish to get attention towards their thread. It's somewhat useful for the Marketplace.