In Chapter 5 of Lord of the Flies, Golding writes that

There was a long pause while the assembly grinned at the though of anyone going out in the darkness.

Significantly, this line bespeaks of the weakness that lies in mankind when faced with its own shortcomings and dark nature.  Too quickly, it is much easier to laugh and deride the grim truth or find other explanations for what is too uncomfortable a truth.  For instance, the boys try to label the beast as a sea creature.  But, after Ralph gives Simon the conch, Simon explains,  "What I mean is...maybe it's [the beast] only us."

"Shocked out of decorum," Piggy, who has said that he seeks a solution, rejects Simon's intuitive answer since he represents the rational side of man:  "Nuts!"  Piggy's reaction causes Simon to become "inarticulate in his effort to express mankind's essential illness."

When Simon attempts to explain to the boys by using the analogy about "the dirtiest thing there is," his efforts are parodied, and he shrinks back to his seat; symbolically, Ralph peers "into the gloom," the darkness of the boys' minds that will not open to Simon's insightful observation.

The idea that Simon comes up with in this chapter is that the beast that they have been worrying about really exists.  However, it is not a big snake or something else like that which lives out in the jungle and will kill them.  Instead, the beast is inside of each of them.

Simon is trying to tell them that the beast is their own evil instincts.  However, he is unable to get through to them.  When he tries to explain it to them, they make a joke about feces and then start laughing and ignore the point Simon is trying to make.