LMB is the same as left-click or lclick, meaning "click the left mouse button".
MMB is the same as middle-click or mclick. NOTE: If you don't have a middle mouse button, press Alt+Right-click.
RMB is the same as right-click or rclick, and is sometimes called the "context menu" or "options menu".
TouchDesigner works with the three-button mouse.
See also RMB Menu.