On Google Ads, ad reach is an estimate of the number of people within a location target, based on signed-in users.
You can use the provided reach numbers to get a rough idea of how many people your ads could reach within a geographic area. With reach, you can estimate how many customers can see your ads in Michigan or Montana, or compare the number of people who might see your ad in Detroit and Ann Arbor. You can use those estimates to decide whether you've targeted the right areas, or if you need to select other areas that may provide the right level of exposure.
Learn more about ad reach in the Google Ads Help Center.