What does the white dot on the blue bar mean in the weather forecast of Apple mobile phone 10?

"AQI is an air quality index, which refers to the expression of air pollution degree and air quality status based on six parameters such as fine particulate matter, inhalable particulate matter, sulfur dioxide, nitrogen dioxide, ozone and carbon monoxide. (The well-known PM2.5 and PM 10 are inhalable particles). Air quality index is simply data that can quantitatively describe air quality. Aqi describes the degree of air cleanliness or pollution and its impact on health. The larger the value, the higher the level and category, and the darker the color, indicating that the more serious the air pollution is, the greater the harm to human health, and it is suitable for expressing the short-term air quality status and changing trend of the city. "