This feature is only available on Craft Pro.
Layouts give you a means to control different navigations on a per-user basis. This can be especially handy if you wish to create a client-specific menu, while not affecting the navigation for site Admins.
To create a layout, click the Layout tab from the Control Panel Nav Settings page. You cannot alter the default layout, which as it suggests, is what users will see if you don't specify a layout.
Craft Pro installs will be able to assign new layouts to user groups, while Craft Client installs will only be able to select the Client account. Craft Personal does not include layout functionality.