YouTube Go app will show a preview of the video and the video size in MB before you start playing it. You can also save these videos in your device, and later share it with other offline. The preview will include thumbnails of the video from different points. This will give users an idea about the video and what is in it even before playing it. It also shows the size of the video before playing it, it will help users who have limited data bandwidth.
The offline saving feature was also available in the main YouTube app for some time now, but sharing that video is new. If someone has the same app, you can share the downloaded video with other entirely offline.
The YouTube Go app will show recommended videos not only based on the country, it will also check what others in your locality is viewing. The option to download video offline was enable in YouTube app back in 2014. It allowed users to watch videos even if there is no data connectivity, but we were not able to share that videos with others.
This app is not an update on the existing YouTube app, is entirely new. Google will roll out the app in the next couple of months.