What does that mean? Well, you'll be able to download the respective vid to your phone, and play it with no Internet connection in sight, how cool is that, on both Android and iOS devices.
The fly in the honey? You will only be able to do it for a fixed amount of "short time," and it's not exactly clear how long at this point. Still, it will probably be enough to snatch a couple of music videos and listen to them in the car or the tube on your way to work, or grab a quick car battery replacement video on the way to your garage, and sometimes that's all you might need.
That seems to be exactly what Google is gunning for, as the post states "so your fans’ ability to enjoy your videos no longer has to be interrupted by something as commonplace as a morning commute."