Media Center makes extensive use of a common User Interface "widget" known as a Disclosure Triangle. These can be single-clicked and they will typically "open" to reveal additional items hidden underneath. If the particular disclosure triangle is already open, clicking on it will usually re-close (collapse) the item.
These are used heavily throughout the interface of MC, and it is important to understand how they work. They are used in the Tree View to show "child" items under each View. They are used on the dividers which separate the different "panels" of MC to show and hide those panels, and they are used extensively in the Options dialog.