Complex sentence: The term'passive' in this instance means the PIR does not emit any energy of any type but merely sits'passively' accepting infrared energy through the front of the sensor, known as the "sensor face".
Split into simpler sentences: The term'passive' in this instance means the PIR does not emit energy of any type but merely accepts incoming infrared radiation. Infrared radiation enters through the front of the sensor, known as the "sensor face".