As in, bump the thread to the top of the heap. An older post, with few responses or updates or due to increase of traffic, can quickly fall to page 2 or 3 and near obscurity. A 'bump' post just repositions the thread to the top and to visibility.
If you click the 'What's new' section, it shows the posts members are currently posting on. For sometimes for classified ads or ones which needs more member interaction, a 'bump' puts its on top to enable a better viewership. Also I believe there might be users following other users and any posts they comment on, might show up in their feeds. At least thats my understanding.