If you choose to enable Improve Maps, Apple will collect the GPS coordinates obtained through the Significant Locations feature on your device and correlate them with the street address associated with your Apple ID. This will enable Apple to better approximate the geographic location of that and other addresses. Apple will only retain the resulting coordinates in an anonymous form to improve Maps and other Apple location-based products and services.
You can disable Improve Maps or Significant Locations at any time on iOS by going to Settings > Privacy > Location Services > System Services, then tapping to turn off Improve Maps or tapping Significant Locations then tapping to turn off Significant Locations. On Mac, you can turn off Significant Locations by going to System Preferences > Privacy > Location Services, clicking the lock and entering your password, clicking Details next to System Services, then deselecting Significant Locations.