In "The Crucible" Giles says, "Think on it now, it's a deep thing, and as dark as a pit." What is he referring to, literally and symbolically?

This quote can be found in act one, and it comes during a conversation where Proctor, Putnam, Parris, and Corey himself were all arguing over land boundaries, salaries, subjects of sermons, and basically everything else.  For the literal meaning of the quote, Giles Corey is referring to lawsuits--everyone in the town has been suing each other for a long time now, over tiny, petty disputes.  Corey mentions this, and questions it.  Right before the line you have above, he says,

"It suggest to the mind what the trouble be among us all these years...Think on it.  Wherefore is everybody suing everybody else?"

Wherefore means why, so Corey is asking why are we suing each other?  He is alluding to the fact that everyone has little grievances, jealousies, issues, contentions, fights, tiffs, and discontents with people in the town, and they express that contention in the form of lawsuits.

More figuratively speaking, symbolically, he is referring to the underlying current of contention and bitterness that people feel for each other in this town.  Corey is saying that their hatred and jealousy for each other is a deep, dark pit that is threatening to swallow them all.  A pit is a snare, a trap that catches people unaware and imprisons them.  It is usually hidden and a surprise to the person caught in it.  This is the same as the contention between people in the town--on the surface it looks like some petty fightings, no big deal, but it will eventuality ensare everyone in the town, leading to their destruction.

In "The Crucible," unfortunately, people turn away from suing one another to express grievances, and turn instead to accusing people they have issues with of witchcraft.  Accusations of witchcraft replaces lawsuits as the new "in" way to get back at your neighbor when you are mad at them.

I hope that these thoughts help a bit; good luck!