This command displays the opening Help-Contents screen, from whence you can branch to all parts of the help system.  To obtain information on a particular command, for example, click on Menus, then on the name of the menu the command is located under and then on the command name.  Alternatively, click on Toolbars and approach it from this route (assuming the command is represented on one of the toolbars).  Wherever you are in the help system you can click the Contents button to return to the opening Contents screen.

The help system in L3DD doubles as the L3DD manual, and is very comprehensive.  It is also replete with a large number of illustrations.