The video was posted by Windowsblogitalia, who first found it on the Chinese Youku video site (China's version of YouTube) posted by the Wpxap site. The clip is in Chinese, but it does show the new design for the Windows Phone Store in this build, which has the version number 12544. It also shows the universal Office apps like Word and OneNote that Microsoft previously said would be released to Windows Insider members by the end of April. In addition, there is also footage of a way to customize wallpapers for the Start screen.
Microsoft has not announced when the next Windows 10 for phone build will be launched for Windows Insiders, although their reveal that universal Office apps will be out by the end of the month would seem to be a clue as to their release date.