As the title before this very sentence says, this is the outline of the whole article/"book". Since it is a web page, I link to the corresponding chapter instead of mentioning the page number. The very first chapter starts with explaining what a function is and what I mean by "one-way" and "linear". If this is beyond your attention span, then this article isn't for you. (PS: I'm the author of the article. I'm also aware that it's difficult to get an overview on mobile, so I'd suggest reading the article on a laptop or tablet.)

Those are all explained in the article. This sentence describes what you will understand by the end.