Pir sensor
This is a PIR ( Infra Red ) sensor ; this device is detecting movement of people by sending a signal on its output. This sensor is generating a +3.3V signal on output when a movement is detected. The duration and the sensitivity can be parametrized.
As you can see on this photo, you have 3 things you can configure on this device that will impact its behavior. The purpose of this post is to describe the use of each.