Most of these commands have to do with bookmarks, the little markers in your Movie Track that let you jump among important spots in your movie without any scrolling or hunting. Bookmarks are described in detail in Section 5.8.1.
Add Bookmark creates a little green diamond marker in your Movie Track at the position of the Playhead. Keyboard shortcut: -B.
Previous Bookmark/ Next Bookmark make your Playhead jump backward or forward to the next closest bookmark you've already created. Not nearly as useful as their keyboard equivalents, -[ and -].
Add Chapter Marker creates a different sort of marker: a tiny yellow diamond that denotes the beginning of a scene or chapter, as it will appear on a DVD's mainmenu screen once you export your movie to iDVD. You can place chapter markers only at the very beginnings of clips.
Remember that you can type names for your chapter markersand access a second way to add and delete themby clicking the iDVD button beneath the effects and sounds panel. See Section 15.4 for more on chapter markers. Keyboard shortcut: Shift- -M.
Delete Chapter Marker deletes whatever chapter marker you've clicked in the Movie Track. Keyboard shortcut: Option- -M.
Show Podcast URLS . in Section 13.2.1, you can read about the little clickable Web links that iMovie lets you superimpose on your video. You create them on the Chapters pane of iMovie, where you can assign a Web address (that is, a URL) to each marker. If you have no intention of adding Web links to your movies, there's no reason you should have to stare at those gray "Link URL" placeholders beneath each chapter name ; choose this command, turning its checkmark off, to hide them. Keyboard shortcut: Option- -M.