The idiom "deer caught in the headlights" refers to a person who is frozen or paralyzed with fear, uncertainty, or shock. It's based on the image of a deer being startled by bright lights and unable to move as it stands motionless in the path of an oncoming vehicle. The phrase is often used to describe someone who is overwhelmed or unable to respond to a situation due to fear or surprise, much like a deer caught in the headlights of an oncoming car.