This is "snippets" for the Cloud 9 IDE.
Add this plugin to the editor and you'll be able to add code completion snippets the editor.
Currently the plugin creates a snippets folder with a sample snippet in it that you can activate by typing html5 and then Command-Shift-U (Ctrl-Shift-U on PC).
The "html5" text will be replaced with the contents of the html5 file in the snippets folder.
To add a snippet, just create a new file in the snippets folder named with something that is easy for you to remember. Type that name in the editor, press Command-Shift-U and the contents of the file will replace what you just typed.
TODO: add some additional snippets, and a nice UX for creating snippets. Might be nice to make the snippets a web service so that they can be shared and don't rely on local files to work.