Sara said:
While I don't know for certain, here are two possibilities:

1) It's part of a bigger promotional campaign. Maybe there's another billboard that explains it further down the road, or maybe at some later date, it will have some text added. For the moment, it's just there to get people curious.

2) In theory at least, it is possible for just about anyone with the money to do it to rent a billboard and put just about anything they want on it (though I'm sure there are some rules to it). So you could possibly be looking at someone's artwork. No advertising purpose; it just looks cool.
 

Tintros said:
My guess (based on previous experiences) is that it'll be filled in with text later.. most likely making it uninteresting. Remember 'What is the Matrix?'