Because Lobby is built on top of stateless networking protocols, the service needs a way to determine if a host is still available. This is done through heartbeats where the host sends an occasional "keep-alive" message notifying the service that they are still online.
If the Lobby service does not receive a heartbeat from a host in over 30 seconds, the lobby will be marked as inactive and will not appear in query results. If a host sends another heartbeat within an hour, the lobby will become active again. However, if a lobby fails to send a heartbeat for over an hour, the lobby will be considered expired. It cannot be reactivated and will be deleted by the service.
For more information, see the documentation on Heatbeating a Lobby.