Giddy up is a term that comes from horse riding. It is something that is said when you want the horse to go faster. It is done in association with either moving the reins or using your knees to knock the sides of the horse. I don't think that the horses really understand the words ;-)

Precisely it's used to command a horse to go ahead or go at a faster pace, but in slang it means "Let's go!" too.