Okay in short whenever you reply to a post it bumps it up to the first one on the thread shown. “Bump” is just a quick and simple way to do that. If you are replying to a post and don’t want to bump it up you would click “sage”. Hope that helps!
Hello! Welcome. Also, to answer your question, people bump posts so that they go all the way to the top the page because they are probably intresting or important. <3