The app has been updated to include Google Cardboard support. Prior to this, YouTube had Cardboard support but it was only available on the Android version, but it looks like after more than half a year, the update has made its way to iOS devices. The feature will be similar to the Android counterpart so if you’ve used it before, it won’t be too different.
Basically for those who haven’t, when watching YouTube videos users will now have the option to watch it in 360-degrees by pressing the Cardboard button. This is different from regular 360-degree videos as it has been catered specifically to Cardboard headsets. Alternatively if there is no 360-degree option, users will be able to watch it as if they are viewing it on a big-screen TV.
The app has been updated and is already available via the iTunes App Store, so if you haven’t updated it already, maybe you should go ahead and do so.