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.
Tonight I am "Gettin' Up" on that Billboard along I-
70 because I am Out for Fame and
maybe one day I can earn some street credibility by "getting up" on rooftops like Amaze in L.A..