Region monitoring (also known as geofencing) is a way for your app to be alerted when the user enters or exits a geographical region. You might use region monitoring to perform location-related tasks.
Example of geofencing iOS app in swift.
More about geofencing: https://en.wikipedia.org/wiki/Geo-fence
Based on this iOS core location services tutorial http://www.raywenderlich.com/95014/geofencing-ios-swift
You can also checkout more about 'Monitoring the User's Proximity to Geographic Region': https://developer.apple.com/documentation/corelocation/monitoring_the_user_s_proximity_to_geographic_regions