To be clear, you’ve been able to watch 360 videos on iOS since last year. Those you can watch by waving your handset around in the air or tapping and dragging with your finger.
Cardboard support brings full-fledged VR to the table; that menas 3D support in addition to the 360-degree motion.
Of course, there are few real VR videos on YouTube, but Cardboard support can also come in handy if you simply want to watch regular, 3D content on your phone. You could also just watch standard YouTube in a magnified size, although the video resolution will be compromised.
The update is available on the iOS app store now.