Anytime a person walks in front of the projector light, the projector dims the light to protect eyesight. It works thanks to 2 time-of-flight (ToF) sensors underneath the projector lens that measure the distance between the projector and the screen. If they detect something in front of the projector, they'll activate Eye Guard and send you an alert on your phone.
To turn on Eye Guard, press the Projector Settings button on top of the projector or press the Source button on the remote, then go to Advanced Settings > Eyesight Protection > On.