On of the most important usage of your mouse device in combination with RadTreeView is expanding and collapsing items. To expand\collapse an item you just need to click on the expander icon. For more information about the visual structure of the treeview read the topic Visual Structure.
To learn more about the RadTreeView's mouse support take a look at the Drag and Drop topic. There you will find step-by-step tutorials showing you how to perform some of the most common tasks using just your mouse.