Just under a question you will see share this question, bump this question, like and send to a friend. You would click the bump button to bump the question. You bump a question you want to stay towards the top or so itll get more answers or if you like it, or whatever.
A bump is just that a bump, you bump it up so more others will see it and answer it. You can use it whenever you want and to any question you want. I usually bump questions that are really good questions to me or ones I think that need attention but I can't answer. It's totally up to you though. the bump question is right under the question box.