The kernel plays an essential role, as it manages how your phone, tablet, and PC's hardware and operating system.

An operating system is a collection (or suite) of programs that manages and controls the computer. Operating systems have many functions: ...

This IDC survey of 1,025 IT decision makers illustrates cloud strategies at organizations and the role of server operating system environments. Commercial Linux is the only environment that surveyed ...

I think the diagram is just trying to emphasize that operating system functions are usually invoked through a different mechanism then a simple library call.

The course covers nine areas, with the first six being core operating system functions and the remaining three covering more advanced operating system concepts.