The commands in this menu are about processing your document and viewing the results. Some commands have their button on the toolbar.
This command runs lilypond on the current buffer. LilyPond is run via the Console plugin using the LilyPond shell.
This command will run convert-ly on the current document. It will only work if you have a version tag in your document (like \version "2.4.2").
This command will invoke lilypond-book on the current file. In the appearing window you can set the options easily. Use the tooltips to get help.
You can view the processed score with this command, using the embedded PDF viewer. See Chapter 5, The dockable PDF viewer for details.