A right click is a the process of pressing a mouse's secondary button (usually located on the right side) rather than its primary button (usually located on the left side).
Most Windows computers have mice with two or more buttons so right-clicking should be intuitive and natural.
Many Macintosh computers have Apple mice with only one button. To right-click on a Mac which only has one mouse button, hold the ctrl key while clicking the mouse.
If your Macintosh has a Mighty Mouse, pressing on the right side of the mouse invokes a secondary click, although this might not be visually obvious.
A right-click is oftentimes a shortcut to find out more about the item you clicked on or options that you have within the context of the item that was clicked. Think of the primary (left) mouse button as the action or enter key on the mouse and the secondary (right) mouse button as an information button.