This is a technical feature, so don't worry if you don't understand.
It's for people who use Little Outliner who need to edit the elements of the <head> section of an OPML file.
You must be running version v1.64b or higher. The version number is in the upper right corner of the screen. If you need to update, reload the page.
To edit the <head> elements, choose Edit OPML head in the File menu. A dialog appears with the existing head elements. Not all of them however, just the ones you can edit.
To add a new head element, click the + button in the lower-left corner of the dialog. When you're done, click Save, and your changes are saved in the OPML file for the outline.
To view the OPML file for an outline, make sure it's public, using the Get public link command in the File menu. Then choose View OPML from the File menu. That will open the OPML file in a new tab.
When would you use the feature? When you want your OPML file to plug into an app that gets options from the head elements of the outline. That's how the Listicle software, in development, gets values such as the author's name, the URL of the background image, etc.
This is a lot like editing the attributes of a headline using the Suitcase icon, but these are attributes that apply to the whole file, not just a single sub-outline.