Offline Viewing means that you can download a video to watch at a later time when you may not have an active Internet connection to watch said movies and TV shows. This feature is ideal for travel or other low connection situations when you want to use the streaming service.
This new service comes as a surprise to many after Netflix has said it would never offer offline viewing. But the fact that Amazon Prime Video has offered offline viewing to members for more than a year, while YouTube Red subscribers also have access to videos in offline mode may be the prime reason why they have decided to do so as well.