Outlining is that little [PLUS] or [MINUS] sign in the left gutter of Visual studio. You see these around the manual regions that you create – and around natural code blocks like classes, methods, etc. New in Visual Studio 2010 is the ability to create Ad-hoc regions, without the #region command. It’s rather nice, because it doesn’t change the source code for others, it’s your own private little “hide this code” trick :). This short 4 minute video gives you a great introduction.
http://scottcate.looplogic.com/dev315-outlining