Signage offline caching affects carousel signage screens, allowing them to operate when you drop connection to the internet. This is particularly handy to keep the service running and avoid the dinosaur game appearing whilst you're trying to serve customers. 

Signage Cache will cache images you've uploaded to your screens, and store them ready for playback. This will work even if the page is refreshed whilst you're still without an internet connection. A small toast notification will appear in the top right corner to warn you that offline content is being shown, and the page will automatically reload itself when the connection is restored - destroying the error.