ForumCommunity

What it means
"Bump" is usually an abbreviation for Bring Up My Post.
To bump a thread is to post a reply to it purely in order to raise the thread's profile. This will typically return it to the top of the list of active threads.
This practice can be very annoying: a user would have to read many messages without content while reading. For this reason, in many forums this practice is prohibited or specially regulated.

On ForumFree/ForumCommunity/BlogFree
This is especially forbidden. On the support forum, for example, is prohibited bumping within three days from the post. There are not permitted either the "group bumpings": this is an incorrect way to give visibility to your thread.