It is to leave your name or tag up somewhere. In today's graffiti world it is a generalized term signifying earning a reputation by placing your moniker somewhere whither seen or not. Traditionally "to get up" meant to place
one's graffiti name in a high and visible location like a Billboard, Rooftop or High Rise i.e.
literally "Getting Up" somewhere high and visible in a sometimes
difficult spot for notable and street fame.