Bumping a topic is simply bringing it to the top of its forum. BUMP stands for Bring Up My Post.
Posts whose only objective is to bump a topic, and which contain no information pertaining to the topic(s) are SPAM and will be treated as such.

Another technique sometimes used is to bump all the topics in a forum just to get more posts. This is Flooding, and is SPAM, even if the posts in question contain information pertaining to the topic(s).