What does Offline mean?
Offline means that your camera has been disconnected from your Wi-Fi source and is now the equivalent of "off" since the camera is not designed to function unless it is connected to the internet. When your camera is offline, you can't view live view, can't change and configurations and your camera is not recording.
How to know if your camera is offline?
Look at the back of the camera itself:
If the middle and bottom light are green your camera is online.
If the lights are any other color, than your camera is offline.
How to tell if your camera is offline from the app:
If your camera is offline, you will also be able to tell that it is from the app.
Your camera will not be listed in the "Online" menu
But if you push "All", you will see them listed in the "All" menu:
Note: Also notice that to the left of the cameras names, you see a grey caution sign
How to tell if your camera is offline from the computer:
If you look at the screenshot below, you will see all cameras that have the grey x icon next to their names are offline. The only camera "online" in this screenshot has the green icon next to the camera name.