How it works? If someone types keyword/location field, and select a distance value, the address will be geocoded to latitude and longitude and all properties that are in the selected radius of that point are returned in search results.
If you want to enable Radius Search in your theme, you need to create another Google Maps API Key without any key or domain restrictions. This key will be used on server side geocoding and it won’t be public.
Once you created new API Key, paste it to Realteo Options → General Options → Gooogle Maps API key for server side geocoding
Then go to Realteo Editor → Search forms, choose form where you want to add it and drag&drop Radius Search field from the available searchable elements.