Run Verification on Existing Data is a manual process that runs each time you save the Website Options Page with this option selected.
Upon saving and exiting the Website Options page, the process will check all user address fields and generate a Latitude and Longitude for the address. If it cannot verify an address for a user, that user will be prompted to update their address the next time they view their Basic Information page.
At the time this process runs, it will count as a unique API call for each user and may incur charges for the Google Places API usage.