Win 10 S is the Windows 10 version of Windows RT. Windows RT was a limited version of Windows 8 designed for the ARM platform that could only run apps from the Windows Store, not full desktop applications that were hardcoded to the Win32 API in regular editions of Windows.
The idea of Win 10 S has not been well received by parts of the developer community who accuse MS of locking down Windows and turning it into a less open platform.
To which the obvious reply is to take a closer look at supporting Linux as the platform of choice in years to come.