Hence means "for this reason," or "therefore." One Hollywood starlet is a huge fan of tiny dogs, hence the Pomeranian puppy peeking out of her purse.
Hence not only means "therefore." It can also mean forward from this time. Nine months hence, a baby that has just been conceived will enter the world kicking and screaming. These days, you're more likely to hear the word hence in a movie set a hundred years ago than in casual conversation because it has a very old-fashioned ring to it.