About this tutorial:
Video duration: 00:04:51
This article is explaining the difference between Modern UI Pages and Classic Pages of SharePoint. Modern UI is latest experience of O365. This is unified, seamless, responsive user experience of O365.
Whereas classic UI is previous SharePoint experience. Current version of SPO does provide support to both classic and Modern UI.
Detailed Article: https://social.technet.microsoft.com/wiki/contents/articles/51286.sharepoint-online-modern-ui-vs-classic-ui-pages.aspx