Offline-first describes software built to run without a network connection, keeping its data on the device and syncing later only if you choose to. For an open house sign-in app it is the difference between a form that always works and one that stalls when the venue Wi-Fi drops.
Vacant listings, rural properties, and buildings with locked networks are exactly where connectivity fails, and they are common open houses. An offline-first app makes zero network calls during a sign-in, so a dead zone changes nothing about the flow.