Keyword Manager Preferences
This pane allows you to add and remove keywords that will show up in the Common Keywords portion of the Keyword Manager.
The Keyword tree starts with "Keywords", which can not be removed and is not shown in the Keyword Manager. It is there to represent the root of the keyword tree.
Clicking "Add Keyword" adds a keyword as a child to the currently selected Keyword. This allows you to build a heiarchy of keywords.
Clicking Remove will remove the currently selected keyword.