The calculation of visibility is complicated and shows the visibility of your domain in the Google index.
For this purpose, over one million keywords are queried and evaluated every day.
The results are weighted by position and search volume for the keyword. So 10th place for a very high-traffic keyword such as "real estate" results in a higher value than 1st place for a seldom searched keyword such as "buy apartment in Cologne city center."
These values are added up for each domain and result in the visibility index.