Most computer mice have at least two mouse buttons. When you press the left one, it is called a left click. When you press the one on the right, it is called a right click. By default, the left button is the main mouse button, and is used for common tasks such as selecting objects and double-clicking.
The right mouse button is often used to open contextual menus, which are pop-up menus that change depending where you click. For example, if you right click on the desktop, you may see a menu pop up that includes "Change View Options" and "Change Desktop Background." If you right click on a folder, the menu might include options such as "Open" and "Properties." Certain programs, such as video games, may use the right click to perform other functions, such as firing a secondary weapon in a first-person shooter. Most programs, however, use the right click to open contextual menus.