This is a Home Page component that is embedded on the side of most pages.
You can run custom code from the sidebar using the general following steps:
Click "Save" without unchecking the HTML view option.
Next, Setup > Customize > Home Page > Home Page Layouts, create or modify an existing layout to use the new feature. Also, assign the layout to profiles that should be able to see the feature.
Finally, Setup > Customize > User Interface, make sure you are showing custom home page components on all pages.
In this example, the source was uploaded to a Static Resource called myLibrary, so you'd want to do the same if you chose to use a Static Resource. You could also host it as a Document, an Attachment, or on some external server (but I would strongly recommend it be served from an HTTPS link).
I myself had implemented Multilingual Field Level Help some years (May 2007) before salesforce.com released their own version; it used the same logical concept of embedding code in the sidebar. Unfortunately, it's an S-Control and uses obsolete browser technology, so it doesn't function, but it was a decent proof of concept.