As far as I know, the editor.css file is included by the CMS (CMSMain.php), and it's used to load styles for the WISYWYG Editor. Usually, editor.css can be found in mysite/css or the themes/yourtheme/css directory.
The @import statement is a CSS directive to load other css files. This statement is part of the CSS standard. Here's an article that describes it: http://www.cssnewbie.com/css-import-rule/
So to summarize:
1) CMS includes editor.css,
2) editor.css uses @import statement to import the typography.css styles
At least that's the common behavior. Of course you can edit the editor.css and import other stylesheets too.