inline and defer CSS

I read the tutorial, but i doesnt understand it.

In tutorial i read: “paste the block of “above the fold CSS” in the input field (text area)”

What means “the block of above the fold css”?

I know what above the fold css is, but i dont know what i have to add there…

Should i add the css files that i want “define” as “above the fold” (that ones i want to defer)? Like this:

http://www.mysite.com/wp-content/plugins/simple-lightbox/client/css/app.css?ver=2.3.1

Have i add one css file per line??? Thanks for any help…

https://wordpress.org/plugins/autoptimize/