BUMP means Bring Up My Post! When someone posts "bump" in a reply to a post, they are bringing the whole thread to the top of the group so more people will see it. The posts on the group are in order of when the last reply was added.
There is a list of commonly used acronyms in the CafeMom Newcomer's Club:
\The CafeMom Admin Team
at 3:46 PM on Apr. 2, 2009