It is a lightweight version of the latest operating system from Microsoft. It is going to have many restrictions for users, for example, it will not allow installing apps from other sources than the Windows Store.
Before you install any software, the system will check whether it is found in the official marketplace. If not, Windows 10 S would not let you complete the installation. That is why most of the traditional users don’t like this idea and are not going to become the users of Windows 10 S.
However, this is a good decision for lightweight platforms, such as Microsoft Surface Go and other similar devices. It also provides the higher security because you would not be able to install viruses and other unreliable programs.
With the October Update of Windows 10, the users of Windows 10 S will also get new features. For example, the Your Phone app, that will help to synchronize mobile devices with desktop computers. If you are using Windows 10 S and you want to get these updates, just turn on automatic updates and wait until they will be installed.
There are plans to let users easily switch from the S mode to traditional Windows 10, but it is not ready yet. You can do it now but you need to open the Windows Store for it.