In groups, it bumps your post to the top of the forum. In Q&A, it doens't do much - if a question gets enough bumps it appears in the popular list, but only if you hit the bump button - putting bump in reply in Q&A is worthless.
at 1:26 PM on May. 9, 2011
at 1:28 PM on May. 9, 2011
when you bump a question in the question and answer area it brings the question up closer to the top again because sometimes they get lost in the list and forgotten about