The main action of Who's Afraid of Virginia Woolf? centers around the vicious battle of wills between George and Martha. Martha is a ruthless opponent, and George doesn't get the upper-hand until nearly the end of the play. After being brow beaten, humiliated, and cheated on, George defeats Martha with four simple words: "our son is…dead" (3.245). Martha reacts to this news by erupting into a bestial howl and collapsing to the floor.
It would seem pretty normal for Martha to react dramatically to the death of her son if she actually had a son. The thing is that George and Martha's son is purely imaginary. When they found out they couldn't have kids, they solved the problem by just making a kid up. Even though he's imaginary, both George and Martha have deep attachment to the boy. Martha reveals the depth of her feeling when she says that he is, "the one light in all this hopeless…darkness" (3.401). The darkness in question is probably her "sewer of a marriage," which she also describes as "vile" and "crushing" (3.401).
This dream of a son seems to be so precious to both George and Martha because it's one of the few things they share. They created him together in order to escape from their "sick nights, and pathetic, stupid days" (3.401). The boy is the one bit of real intimacy that the unhappy couple shares. When George "kills" the son it's like he dropped a nuclear bomb. Now George and Martha are left with no illusions behind which they can hide. By the end of the play, they must stare, unblinkingly, into the charred battlefield that is their lives.