Google will soon launch a new YouTube feature that will allow users to view certain content without an internet connection. Viewers will designate videos to add to their device, which they will then be able to watch offline in case they lose their internet connection. After 48 hours, the content will no longer be available. On-device videos will be available through a specific section of YouTube's mobile applications, according to their Add to Device FAQ:
From the watch page through the functionality, viewers will be able to designate certain content that they can watch for a short period of time when they do not have connectivity. At the time when the user no longer has connectivity, they will be able to watch these videos and playlists through an “on device” section of their guide.
Google's in-stream ads will run with the content, and any views will be added to the total viewcount of the video when the device reconnects. Add to Device functionality is enabled for all content by default, except for videos for rental or purchase. However, content providers can turn this off before it launches. They can also specifically disable Add to Device by certain criteria, including by partner, asset, video, and country. The Add to Device feature is expected to launch at some point next month.
Source: YouTube Add to Device FAQ