How it works: Adwords Estimated Total Conversions with store visit insights

We never provide anyone’s actual location to advertisers. Instead, store visits are estimates based on aggregated, anonymized data from a sample set of users that have turned on Location History. This data is then extrapolated to represent the broader population.

via Inside AdWords: Measure more: improving Estimated Total Conversions with store visit insights.

Background info here:

Location History and Location Reporting data may be used by any Google app or service, including in ads on and off Google. For example, Google Maps may use it to improve your search results based on the places that you’ve been.

Location Reporting: Allows Google to periodically store and use your device’s most recent location data, as well as activities like driving, walking, and biking, in connection with your Google Account.
Location History: Allows Google to store a history of your location data from all devices where you are logged into your Google Account and have enabled Location Reporting. You can turn off Location History at any point, but doing so won’t delete your Location History. It also will not turn off Location Reporting, GPS, or Wi-Fi location services for your device.