Windows 10 S mode is a specialized version of Windows.
Windows 10 S mode gives you the normal Windows experience. It limits you in some things to give the user a more secure experience and thereby limits you some in what you can do.
Window 10 S mode runs like regular Windows 10 but allows you to only use the Windows Edge browser, which Microsoft rates as a much securer browser. Up until very soon, Windows Edge has been a totally different browser than others like Chrome, Firefox, Safari and others which are based on a Chromium browser engine. Not everything works in Edge but beginning later in January 2020 this will change as Microsoft has created a new Edge that will distribute then (based on announcements in December 2019) and then pages should basically work the same on all the major browsers.
Windows 10 S mode limits you on applications/apps that you can install and run. This version of the operating system only allows you to install applications/apps that you got from the Microsoft store. This is the same thing that Google has tried to do with the Android operating system on phones and Apple with iPhones. In all three cases they say they have checked the applications available there to prevent them having malware, etc. include with the products. However, in all cases there have been cases of stuff slipping through, but still some type validation. Google Chrome for instance is not available from the Microsoft Store.
Windows 10 S mode is Windows 10 and has all the experiences you are used to, but with some limitations to attempt to make it a securer and safer version than with the more open regular Windows 10 or Linux.
For the person who uses the computer to browse the Internet and do email they will be fully comfortable with this version. Those who use it for more advanced things like photo editing, doing classes online, advanced gaming it is not a version for them.