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 retain the resulting coordinates only in an anonymous form to improve Maps and other Apple location-based products and services. Your iPhone will also periodically send locations of where and when you launched apps, including the name of the apps, in an anonymous and encrypted form to Apple in order to improve Maps and other Apple location-based products and services.
You can disable Improve Maps at any time on iOS by going to Settings > Privacy > Location Services > System Services, then tapping to turn off Improve Maps. On Mac, you can turn off Significant Locations by going to System Preferences > Security & Privacy > Privacy > Location Services, clicking the lock and entering your password, clicking Details next to System Services, then deselecting Significant Locations.