If you made a post and it down below the first page and has not been considered for reply for some obvious reason, or maybe you want to remind of the existence of the thread for others to see and have in mind for recognition, then you may chose to bump the thread, this will make it go up and appear on the first page, so as other members may see it or comment under it, this should be done also on post that are not spam neither should anyone take the advantage of that to spam up a board.