Windows App Studio was a free, online app-creation tool for enthusiasts and developers. It was used for creating Windows Universal Apps that could be extended and compiled with Visual Studio and submitted to the Windows Dev Center.
After 15 July 2017, only exisiting users will be able to sign in. Two months later, the application editor will be shut down with the service mothballed on 1 December 2017.
There's some more information on the Windows Blog.
It's always a pain when services are shut down but at least Microsoft is providing a path forward.