When using the Super Simple app in offline mode, your most frequently and recently viewed content will be available along with anything you've marked as "favourite". It saves these videos to your device so you don't need an internet connection to view them.
If there isn't as much content in offline mode as you expected, you may want to increase the amount of space you've given to the Super Simple app. If you've already done that, you may need to free up some storage on your device.
To see how much storage your device has available, go to Settings > General > iPhone/iPad Storage. This will give you a detailed breakdown of where your storage is being used.