A passive Infrared sensor(PIR sensor) is an electronic sensor that measures infrared (IR) light radiating from objects in its field of view. All objects above absolute zero emit heat energy in the form of infrared radiation (infrared light). Usually infrared light is invisible to the human eye, but it can be detected by electronic devices designed for such a purpose.
PIR devices do not generate or radiate any energy for detection purposes. They work entirely by detecting the energy given off by other objects.
A PIR-based motion detector is used to sense movement of people, animals, or other objects. They are commonly used in burglar alarms and automatically-activated lighting systems.
Read wiki‘s article on PIR sensors.