emacs

general purpose text editor with programmer-friendly features

Quick Reference Guides

How To

What do C-x, C-c, and M-x mean?

About M-x, the “named command” key

There are lots of commands in emacs that are “named commands”, and not bound to any key by default.
To execute them, you bring up the emacs “command line”, which you do by typing M-x (type escape once, then x once.).