Outlining in this context means to break up a bigger structure in smaller parts. Like break a document up into each individual paragraph, or sourcecode into functions. Each smaller part is then available to be handled in (semi)programatic ways.

For example, you can link one part to appear on multiple locations of the document. Or you can write a script which filters the parts and rearranges them in your desired structure.

The point of something like leo editor is to have theoretically a unified way to do all that.

I was thinking the same as you. It seems to be another term for code folding. It is where you collapse parts of a source file into a single line so you can just focus on the part of the source file that you are interested in.

Like org-mode?

It’s more like the literate programming parts of org-mode except Leo is more of a live view, from what I remember.