This kind of sensors uses the invisible infrared (IR) light to detect objects.*
Depending on how the sensor is built, i.e. if the sensor only contains a receiver (RX) or an emitter (TX) too, we speak respectively about “passive” infrared sensors or “active” infrared sensors.
“Passive” because the sensor only “senses” the infrared light he catches from its environment. However, it is not radiating any infrared waves, but passively accepts incoming infrared radiation.
“Active” because the sensor emits an infrared wave (generally at a wavelength of about 850nm). The reflected infrared energy/light is then sensed by the receiver.
* “Infra” meaning below our ability to detect it visually, and “Red” because this colour represents the lowest energy level that our eyes can sense before it becomes invisible. Thus, infrared means below the energy level of the colour red.