This quote from The Road is apparently a conversation between the man/father and his wife/the mother of the boy. It occurrs in the past, sometime before the father and son are on their journey when the novel begins. In this exchange, the woman wants to leave, and even more, wants to die, because it seems the apocalyptic event has begun and she sees the future as hopeless.
The wife's lines in this brief dialogue emphasize the enormity and desperation of the situation at this point in the world-ending/world-changing event. She refers to a shadowy "They" who threaten the family. She thinks they are in mortal danger from this outside group. Later in the novel, we do see signs that some humans have turned to cannibalism to survive, and McCarthy also uses that as an example of how degraded humanity's moral compass has become in this bleak new reality.
The woman poses the question of why they don't talk about death any more , and she answers her own inquiry by saying it is because death is too close. It's too realistic now. This foreshadows the scene later in the novel when the man is about to die and must try to prepare his son for the fact that he'll soon be alone in this world.
The woman's sense that survival is now "meaningless," that their lives have no real value any more, leads her to use strong language related to desire and adultery. She says her decision to leave the family could make him think of her "as a faithless slut." This is because her leaving the family will be perceived as an act of betrayal, though not exactly of the sexual kind. She talks of embracing a new "lover," Death. Even though the man does not think Death can offer her anything, she indicates otherwise. She feels that Death is a more human option than whatever else is left to them at this point.
The conversation shows us some context (why the woman/mother is not traveling with them) and suggests the gravity of the changes that brought about the post-apocalyptic setting where we find the man and boy. It also indicates that some people, in the face of such disaster, see death as more appealing than what will become a constant struggle to survive.