This passage from Cormac McCarthy's The Road sets up a contrast between reality and dreams. The novel takes place in a post-apocalyptic world that is extremely bleak. The whole world seems to be gray, and the darkness of the physical world is also reflected in the lack of hope and the barbaric behaviors of the humans who are still alive, trying desperately to survive.
In this passage, the man is described as seeing colors and light while he is asleep. Those elements of a past world are no longer present in this post-doomsday reality. The idea of "human love" is a rare concept. Of course, the man and the boy, father and son, carry on their emotional connection, but in the world at large, desperation sometimes leads people to act in ways that perhaps are not motivated by love.
The first line of the passage is also significant in terms of the whole novel. It simply states, "They went on." This line represents the human will to endure, to persist, despite the bleakness and hopelessness of the circumstances. Part of what helps these two characters endure is the idea of the fire that the man says the boy is carrying. It could symbolize hope or purity, aspects of the world that seem lost but could possibly be revived through the boy's survival.
The passage relates to major themes of the novel such as survival in extreme conditions, the role of hope in a seemingly hopeless world, and the contrast between the world before and after the apocalypse.