The three color dots indicate CO2 level in the air. The CO2 level is measured in parts per million (ppm) letting you know when you should increase the airflow of fresh air in the room.
Green – good (below 1000 ppm). Optimal indoor air quality
Yellow – average (1000 ppm). Brain cognitive function lowers by 15%.
Red – unhealthy (1400 ppm). Brain cognitive function drops by 50%