First in, last out is a principal used within the geofencing mode. First in, last out allows you to come and go without worrying about arming and disarming your Arlo cameras. For example, if your roommate leaves the house before you, your Arlo cameras remain disarmed because you are still in the house or geofence. After you leave the house and both devices are outside of the geofence, your Arlo cameras are armed.
For more information about Arlo geofencing, visit the following knowledge base articles: