It is primarily a text about human fears. Pennywise the clown serves as an embodiment of these fears, as he can shapeshift into whatever his victim fears the most. Children are especially vulnerable to their imaginations, which is how Pennywise can so easily frighten the members of the Loser's Club. He uses certain tales that the children have heard, and their own traumatic experiences to terrorize them. In this novel, King shows how every individual has their own complex fears, informed by their unique experiences and memories. For example, Beverly's fear of blood is symbolizes the fear of her impending womanhood.
Related to the theme of fear is the idea that each person has their own struggles and issues to deal with, usually arising from childhood. King shows that many children are affected by the household they grew up in, especially if it involved abuse. A less extreme example is the fact that Bill's guilt has been strengthened by his family's reaction to the death of his little brother. On another level, we see more abusive households, such as that of Henry Bowers, who ends up killing his father after years of abuse. Beverly also comes from an abusive household and marries somebody similar to her father later in her life.
Due to abusive families, bullying from peers, and the wrath of Pennywise, childhood in this novel is far from idyllic. We are first confronted by the untimely death of Bill's little brother George, and from there we consistently see the destruction of innocence due to various factors. In doing so, King subverts the innocent, idyllic idea of childhood that is often seen in literature, instead offering a grimly realistic portrayal of what many children experience. Pennywise the clown is seemingly the embodiment of an evil that existed long before his arrival and represents the corruption and evil that children face when they are confronted with the reality of the adult world.
