Nick describes the billboard standing in the valley of ashes as an old advertisement erected by "some wild wag of an oculist" with a practice in Queens. In other words, Dr. T. J. Eckleburg was either an ophthalmologist or an optometrist, and he must have set up the billboard in order to gain customers for his practice from this area. However, the billboard has fallen into such disrepair that it has dimmed quite a bit, but it consists of a pair of eyes, "blue and gigantic," behind huge yellow eyeglasses. There is no face or nose, just the big eyes looking out over the valley:
a fantastic farm where ashes grow like wheat into ridges and hills and grotesque gardens where ashes take the forms of houses and chimneys and rising smoke and finally, with a transcendent effort, of men who move dimly and already crumbling through the powdery air.
The valley is where the poor—those who cannot afford homes like those on West Egg or East Egg—live, and they are the detritus of industry booming during this era, just like the ashes. They are the workers who pay the price of industry with their bodies. It's a godless place where people suffer endlessly no matter how hard they try to work, as we see with George Wilson.
Later, after Gatsby's car (with Daisy at the wheel) strikes and kills Myrtle (George's wife and Tom's mistress), George tells his neighbor that he knew she'd been having an affair. He'd said to her,
she might fool me but she couldn't fool God. I took her to the window . . . and I said "God knows what you've been doing, everything you've been doing. You may fool me but you can't fool God."
While saying this, Michaelis, the neighbor, realizes that George is looking out at the billboard and that George evidently believes that those billboard eyes are the eyes of God. It is telling, I think, that George conflates God with an advertisement meant to generate money. This seems to symbolize the idea that money is king—it is like God to America of the 1920s—an indictment of society at large. Gatsby becomes a criminal for money. Myrtle cheats on her husband for money and the privilege that comes with it. Tom's money allows him and his wife to be "careless people" who don't have to pay for their problems. In a way, then, George is right: money is God here. It is all-powerful—it just doesn't care at all about what Myrtle Wilson did.