Well, a few things, actually. First up, at the beginning of Chapter 20, Reverend Hightower sits at his window before the sun begins to set, and marvels at "how that fading copper light would seem almost audible, like a dying yellow fall of trumpets dying into an interval of silence and waiting" (20.2). Some scholars suggest that Faulkner thought that Mississippi sunlight had some rather distinct properties, so one way to think about the title as a way of honoring a peculiar geographical feature – Southern light.
Hightower's final scene is also important in that he finally takes responsibility for being a bad minister and for driving his wife insane, leading him to experience a kind of en"lighten"ment while sitting at the window and observing the changing light outside. In this sense, to "live in the light" might mean to have the ability to see yourself clearly, without such distortions as egotism or a pre-occupation with the past.
Some scholars also note that the phrase "to be light in August" is a Southern slang term for pregnancy; this highlights Lena Grove's role as a character that unites several aspects of the story and brings characters together during her pregnancy and with the birth of her child.We should also let you know that Light in August was originally titled Dark House, and three of the major characters – Joanna Burden, Gail Hightower, and Joe Christmas – are frequently depicted sitting in darkness. We'd say that's a pretty appropriate image for characters that are trapped in the past and in their own egos, unable to experience enlightenment and insight.