In the immortal words of the Great MC Hammer in his singular anthology put to music "U can't touch this".... and I quote,
"Now move. Slide your rump. Just for a minute let's all do the bump. Bump Bump. Bump."
Now back to lermure's question. A bump is you responding "bump" to a thread to keep it live and in the forefront of the numerous threads, posts and replies on Redguide. At the top right of every page is the "You have 52 unread posts". If you click on unread posts a page is generated for you with the most current posts you have not viewed. By bumping you put a current date/time stamp on a thread and people who use the unread posts feature will have that post included.
Playbetter.......