The word "shadow" crops up quite a few times in The Sound and the Fury, especially in Quentin's section of the story. Quentin himself sees his own shadow rising up from the water beneath him while he's in the process of committing suicide. The implication is that such an act is the only way that modern man can deal with the problems of life. The alternative options are not much to write home about. Either one can embrace the world in all its materiality, as Jason does, in which case one becomes grasping and materialistic, or one can be like Benjy, who's mentally disabled, and see nothing but shadows.
In his famous speech from which the title is taken, Macbeth describes life as a "Tale told by an idiot." In Benjy's section of the story, that's exactly what we have. In Faulkner's day it was common for people with mental disabilities to be described as idiots, so here, we literally have a tale told by an idiot, "full of sound and fury" and "signifying nothing."
Whoever's telling the story, Faulkner is suggesting that modern man appears incapable of grasping what is true and enduring. That is because contemporary existence lacks the nobility of spirit of former years, when life had more significance and wasn't full of sound and fury.