Menu extras are the small utility menus at the top-right of your screen that give you quick access to some of the system preferences and other central options in OS X, such as options to select Wi-Fi networks, change system volume, and access Time Machine backups. Third-party developers may also include menu extras to give quick-access to their program's settings and features.
While you can access these menus by clicking them, at times this may be a touch cumbersome, especially if you are actively using the keyboard and not a mouse or trackpad.
For standard system menus (those to the left of the menu bar), Apple supports a hot key to switch focus to the menu bar, but this limits navigation to these system menus and does not allow you navigate to the menu extras; however, Apple does have another option to specifically send focus to the menu extras, after which you can navigate them with the arrow keys.
In OS X Apple refers to the built-in menu extras as "Status Menus," and offers a built-in shortcut (Control-F8) to quickly switch focus to these menus. By pressing this hot key (you may need to include the Fn key), you will see one of the menus highlight, after which you can move among them and expand them with the arrows, and then press Enter to select a function in them.
While Control-F8 works, you might find that reassigning a custom hot key may make access to these menus more intuitive. Previously I covered using Control-Option-Down Arrow as a substitute for the system menus, as it gives quicker access to the menus for folks who are already navigating with the keyboard. Similarly, you can reassign the same hot key but use the up arrow instead, to have this hot key access the menu extras.
The specifics of what to use are up to you and your needs (perhaps use the left arrow for the left-most menus, and the right arrow for the right-most menus), but once set, you can press this hot key and be able to quickly access the menus on your system.
While this feature will work with all of Apple's included menu extras and some third-party ones, it will not work with all menu extras. There are several ways to program menu additions in OS X, and this feature will only activate those that follow the routines Apple uses for its built-in options. Therefore, third-party menu extras like Dropbox, Little Snitch, and antivirus tools may not be accessible via the arrow keys. These might have their own built-in hot keys for some functions, but the only other way to access them is by using the mouse.