The billboard symbolizes how a higher power is watching over the Valley of Ashes and New York City. The eyes on the billboard are directly relating to the theme because although they're fading, they're still there; meaning that some things from the past will transfer to the present but other things, like Gatsby and Daisy, should remain in the past.
"Standing behind him Michaelis saw with a shock that he was looking at the eyes of Doctor T. J. Eckleburg which had just emerged pale and enormous from the dissolving night. ‘God sees everything,’ repeated Wilson." (pg. 170)
Dr. TJ Ecklebergs billboard is a motif. It is mentioned often in the Great Gatsby.