What is beacon monitoring?
You can think of beacon monitoring as a geofence, i.e., a virtual barrier that’s usually defined using a set of geographic coordinates. Moving and out of the area it encloses triggers “enter” and “exit” events, which the app can react to.
In case of iBeacon, the area is defined by the range of one or more beacons. This allows more granularity and precision than regular geofencing—the latter being based on a mix of signals from cell towers, WiFi and GPS. Beacon geofences are also more responsive: “enter” events usually take up to a few seconds to trigger, “exit” events up to 30 seconds. (Regular geofences take “3 to 5 minutes on average”, according to the CLLocationManager’s documentation.)
Best part? iOS will keep listening for those beacons at all times—even if your app is not running or was terminated, and even if the iPhone/iPad is locked or rebooted. Once an “enter” or “exit” happens, iOS will launch the app into the background (if needed) and let it execute some code for a few seconds to handle the event.