I'll explain it in more detail, because I, too, didn't fully understand it. Every time a reply is added to a post, it becomes the first post on the page again. So...replying by saying 'bump' will make the post appear at the top of the page. What it really means is, "I don't have anything to really add to this post, but I don't want it to get 'lost' (meaning end up at the bottom, then to the next page) because no one is replying so I will put in a 'reply' so it goes back to the top". BUMP is much easier to type! So, if there is a question no one has answered yet, if it starts to appear lower and lower on the page, if you 'bump' it up, it will appear back at the top and you have a better chance of someone seeing it and replying to it.
Hope this helps!
Pam