The most common word in graffiti is “fuck.” It often appears by itself — a single word left for others to ponder for decades or else paint over. It is probably meant to express general dissatisfaction with life. An expanded version of the sentiment might read: “I wish to say ‘fuck you’ to every random person who passes here. Such is my anger with the state of affairs in this world and the specific circumstances I deal with in my personal life. Though most people are not necessarily responsible for the things that upset me, I nonetheless hold everyone in contempt.”
It is also not uncommon to see the word “shit” spray painted as a one-word message, which leads me to believe the act of graffiti is often more about exercising the ability to be profane in a public and semi-permanent way than about getting across an idea. At least, I hope so. It seems unlikely that graffiti artists write “fuck” and “shit” as instructions to encourage public fornication and defecation. If they did, they could be much clearer by writing, for example, “shit here.”
There are actual graffiti artists who paint brilliant and thought-provoking murals on concrete pillars, the sides of train cars and so on, but their rebel collages are a bit less common than the scribbled words of the artistically challenged.