Users of
GNU Emacs on modern computers may, for a time, hunt their keyboards for a key called
Meta.
Some keyboards have no Meta key. Most since 1980 do not. Nee, most keyboards from any era do not have a Meta. Emacs can be configured to use pretty much ANY key (not the any key) as the Meta key.
Many GNU setups use the Esc or Escape key as the mythic Meta.
When GNU Emacs tries to describe keystrokes in text, it will use an M- to refer to a Meta+something combination. Of course, Esc isn't considered a "hold it down" shift key to many applications or systems, so trying to coach a newcomer through a key like "M-x C-d" can be painful.