what does it mean?

It's just something people say when they want their post to jump up to the top of the forum. 

I think it stands for Bump Up My Post .... B U M P

The forum threads are listed by timestamp based on the most recent reply.  So someone can push their posting back to the top of the list by posting a reply - either actual new information or often just the word "bump" to bump it back up near the top. (Note "editing" doesn't bump it up, only posting a reply.)

However there are some rules about bumping in the Book Bazaar area where members share deals they are running. Here are a couple of excerpts from the Help Center and the Book Bazaar Rules

-----HELP CENTER --------------

 We ask that members not start more than one or two topics in Book Bazaar per day, and "bumping" of your topic is discouraged, as this is unfair to other members.

• It is perfectly all right to start a topic about your bookshelf and post an update every day, to let members know about any new books or deals you are offering.

----------General Rules in the Book Bazaar----------

....(2) Members may "bump" their topics a maximum of once a day (bumping = posting a new post in a topic for the purpose of keeping the topic on the front page of the forum topic list). Adding a new post with new information is considered bumping. If you edit your original post to update information there, this will not "bump" the topic.