Passive InfraRed sensors (PIRs) are electronic devices which are used in some security alarm systems to detect motion of an infrared emitting source, usually a human body.
All objects, living or not, whose temperature is anything above absolute zero emit infrared radiation.
This radiation (energy) is invisible to the human eye but can be detected by electronic devices designed for such a purpose.
The term 'passive' in this instance means the PIR does not emit any energy of any type but merely sits 'passive' accepting infrared energy through the 'window' in its housing.
An intruder entering the protected area is detected when the infrared energy emitted from the intruder's body is focused by a Fresnel lens or a mirror segment and overlaps a section on the chip which had previously been looking at some much cooler part of the protected area.
That portion of the chip is now much warmer than when the intruder wasn't there.
As the intruder moves, so does the hot spot on the surface of the chip.
This moving hot spot causes the electronics connected to the chip to de-energize the relay, operating its contacts, thereby activating the detection input on the alarm control panel.