The upper right corner contains your most frequently-used system controls. You'll be used to this if you're used to a Mac; on Windows this was your System Tray in the bottom right of your screen, only a lot less annoying.
There's not much to say about the Volume Control. Adjust your system sound by sliding the volume up or down.
If you choose Sound Settings, a sound preferences window opens, where you can choose the alert sounds and control different devices (including the microphone in your computer, if you have one).