"I think maybe they are watching, he said. They are watching for a thing that even death cannot undo and if they do not see it they will turn away from us and they will not come back." To whom is the man referring? What could be a thing that death cannot undo?
In The Road, the man and his son have many awful experiences, despite the man's best efforts to protect his son.
There is quite a philosophical edge to the man's words when he refers to them "watching for a thing that even death cannot undo." The man has seen his fair share of the effects of death and coupled with his personal experiences of death and the pain of losing his wife; for the man, death extends the personal "hell" in which he and his son exist.
The man is trying to look for the hope but is saying that death cannot undo the hurt, the devastation, the situation and that relief is temporary.
Who is dead and who is alive? Sometimes the distinction is hard to make! In this world, life itself has almost become death - for some. All hope is gone! The man has his son to get him through. His life does have some purpose, although sometimes he does wish that he could "turn away" and "not come back."
There are many for whom survival is the only motivation and, unless they can find something more, unless there is something to "watch" for, they are doomed. Death cannot undo hope or purpose. When he dies, he must be sure that his son has found purpose and can go on, otherwise, in death , he will be haunted by guilt and a feeling that he did not do enough for his son.
“Listen to me, he said, when your dreams are of some world that never was or some world that never will be, and you're happy again, then you'll have given up. Do you understand? And you can't give up, I won't let you.”
In the end, death only brings relief if your purpose has been served. Otherwise, "they" will spend eternity looking for the "thing" that death cannot undo.
This is a misreading! "They" does not refer to the man and the boy, but the four people with whom they've had a recent encounter (witnessing the infant they've barbecued and eaten). The man fears they're watching the house.