By spraying it, according to the story line, Carl Johnson (protagonist) announces his come back to rival gangs. What is done during spraying is, Carl Johnson (you) paints Grove Street Family's graffiti over the graffiti of other rival gangs in the Los Santos. This is another side-challenge given to you apart from many other challenges (story missions, etc) of Grand Theft Auto SA. This spraying starts during the Tagging Up Turfin which you're informed about it.
Just like we collected hidden packages in Vice City and similarly we have to spray tags. Similarly, we have those type of things in Grand Theft Auto SA. I remember that in GTA Vice City we had to collect hidden packages. TAGS Map, Locations, Spray, Walkthrough, Reward, Collectables, Guide, Missions game. This post is on 'GTA San Andreas TAGS' for playing game on PC, Xbox and PS2.