I found the description pretty good, but maybe an extra 'headline' might be helpful:

What it does: Use emacs to input text in any program at any time

How it work: A keyboard shortcut of your choosing...

Prefer to use Emacs to edit text? Now you can use it anywhere. Press a keyboard shortcut of your choice, and a new Emacs frame (window) will be opened. After you’re done, just save and the text will be inserted in the original input box.

(*disclaimer: written by an avid vim user, and not a native English speaker. May need rephrasing to be better Emacsese or English.)