The modifier key for Macs when using Terminal (the shell) is set by default to ESC [escape].  It's pretty annoying if you are familiar with using just about any other platform for working in a shell... almost as annoying as learning to use the command key (AKA "Apple Key") as a recovering Windows user.

Below is a screenshot of the Terminal Preference panel and the quickest way to be able to transition from a standard PC keyboard.