This is the passage from Chapter 1 to which your question refers:
In dreams his pale bride came to him out of a green and leafy canopy. Her nipples pipeclayed and her rib bones painted white. She wore a dress of gauze and her dark hair was carried up in combs of ivory, combs of shell. Her smile, her downturned eyes. In the morning it was snowing again. Beads of small gray ice strung along the light-wires overhead. (25)
Shortly after the man and boy pass the barn with the three dead bodies in it, the man dreams of his “pale bride,” his wife who committed suicide and left him alone with their son.
It’s a perplexing dream: is she a temptress? A ghost? A memory of their wedding day? She is certainly deathly—her ribs showing. She also seems to be emerging from the landscape; in his next thought he remarks on the whiteness of his surroundings upon waking. Snow began to fall as he slept. Because she materializes out of his environment, “out of a green and leafy canopy,” she seems inescapable. One might argue that she is the embodiment of his secret desire—to give in to death and follow her.
Directly after this dream the man comments on the importance of nightmares:
He said the right dreams for a man in peril were dreams of peril and all else was the call of languor and of death. (26)
It is hard to tell which category this dream falls into, bad or good. Dressed in white, she is his “bride,” and we can feel his longing for her. With her smile and “downturned eyes,” she is beautiful. But she also appears to us beautifully terrible, dangerous even. She is the shadow of his wife when she was most alive to him, on their wedding day. She is a specter of one of his happiest memories, sent forth to tempt him. Because of this, we might discern that this is a painful, bad dream for the man.
If that is the case then this dream merely signifies his will to survive, however futile that might seem. It echoes his original decision not to follow her into that cold night when she committed suicide. Yet her connection to the snowy landscape also tells us that this dream, or at least the temptation it signifies, will relentlessly follow him along his journey through this nuclear winter.