Hijack - to join a thread and start talking about something totally irrelevant to the subject. That may be asking an unrelated question or starting a silly conversation with another Hubber.
Bump - to bump a thread means to push it back up to the top of the forum list. There are good and bad reasons for doing that.
Say you asked a serious question and no one answered it, so it's getting buried on page 3 of the forum. You might write another post, even though you've got nothing further to say, just to bump it back up nearer the top so people will notice it. That's OK if you've given people a reasonable time to answer. I've seen people bump threads an hour or two after they've asked the question - which comes over as impatience!
The bad reason for bumping is if you've started a thread because you have a bee in your bonnet about something, or you've written something humorous and no one's paid attention - then it's just annoying!