The index provides you with an alphabetical list of key points in the documentation. Clicking any of the keywords in the index displays the appropriate subject in the right panel.
You can search the index by typing a query into the textbox at the top of the left panel. When you click the View button, the left panel automatically scrolls to the first instance of the queried keyword, whose page is then displayed in the right panel. You can click the Next button to move to subsequent instances.