It's something a webpage can access on it's backend (the database of ssid/bssid and their locations) and something your computer uses(the access points its seeing) to locate you in the lack of a gps receiver and saying yes to the browser seeing your location.

Browsers have geolocation api, webpage uses it. Then browser utilizes underlying OS capabilities to determine the location. On the phone it could use GPS, regular PC may relay on SSID of wifi networks, etc.