The Overall Index is developed by Kaiterra to give you an at-a-glance, complete reading of your indoor air quality. It combines readings of both PM2.5 and TVOC concentrations. Based upon the AQI standards you chose in your Settings, the Overall Index calculates PM2.5 and TVOC concentrations independently and displays the higher of the two values, so that you know which pollutant to focus on.
This is similar to how the U.S. EPA calculates AQI, by looking at a range of pollutants and showing the pollutant with the highest readings. You can choose which AQI standard to use in the Kaiterra app.