You can open this dialog by choosing the command in the Outliner menu, or clicking on the Suitcase icon in the left margin.
Each headline in an outline can have any number of attributes attached to it, invisibly. The attributes are saved with the outline, and move with the headline when you reorganize or copy and paste.
Attributes serve as hints, often to the content management system, on how to interpret the outline and its subheads.
For example, the type attribute tells the CMS how to render a page. The created attribute says when the headline was created.
To add a new attribute, click on the + button in the lower left corner of the dialog. A new line will open. Enter the name of the attribute in the left column and its value in the right column.
To delete an attribute, click on the trashcan icon to the right of the attribute.
To edit an attribute, click on its name or value, and use the normal editing commands.
Your changes aren't permanent until you click on the Save button.