Build from scratch
It talks about Build From Scratch. What does it mean in simple languae please?
Is it necessary for us as Qt/QML/C++ programmers?
And how to make such a simple program ("Greetings from Qt 5") using a step-by-step guide please?
Build from Scratchmeans starting from the very beginning / zero.
The referred section just shows how to build Qt from source. This isn't needed for most users, unless you have a special reason and know what you are doing ;)