"Bump" is a response on Facebook and other threaded sites intended to bring the post back to the top of the list. Some claim that it is an acronym of "bring up my post," but this explanation was probably invented after the practice of bumping became popular.
Many forums or other social media sites, including Facebook groups, are organized chronologically by the last comment received on a post. To increase potential viewership, users may add a "bump" comment to their posts (or another post they want more people to see) to put it back at the top of a list. Although the algorithm for the Facebook newsfeed is more complicated, more comments and user interaction generally increases the potential audience for an individual's post as well.