It tells the air quality index(AQI) around you.
The Air Quality Indexes are based on PM2.5, PM10, Ozone, NO2, SO2 and CO hourly measurements.
The table below defines the Air Quality Index scale as defined by the US-EPA 2016 standard.
101~150: Unhealthy for Sensitive Groups
151 ~ 200: Unhealthy
201 ~ 300: Very Unhealthy
301 ~ : Hazardous
Air Quality Data is provided by the World Air Quality Index (AQI) Project.
To know more about Air Quality Index, check the following site.