NPC stands for Non-Player Character, a term originating from video games to describe characters controlled by the game rather than by a player.
In a broader sense, the term is used to describe people who seem to function on autopilot, following social conditioning, repeating established beliefs, and reacting to life without much independent thought or self-awareness. In one of my books about Thoth, I explore this subject in much greater depth. Below is a short, clear summary by Inspired Channel: