In Emacs, what does "M-x command" mean?

In Emacs, "M-x command" means press M-x, then type the name of the command, and then press Enter. The M stands for the Meta key, which you can emulate on most keyboards by pressing the Esc key. For more information on Emacs keystroke naming conventions, see In Emacs, how are keystrokes denoted?

The default function of M-x is the command execute-extended-command. This command allows you to run any Emacs command by its extended (i.e., full) name. If you can't remember all of a command's name, you can type in some of it and then press Tab and the Spacebar to complete it, or press  ?  for a list of possible matches.

Note: Your system administrator may have bound a different key or key sequence to invoke execute-extended-command. A function key labeled Do is a good candidate for this.

This information comes from the Emacs FAQ.

At Indiana University, to get support for personal or departmental Linux or Unix systems, see At IU, how do I get support for Linux or Unix?

Also see:

• For Emacs, where can I get documentation?
• For Emacs, how do I get online help?
• GNU Emacs Quick Reference Guide
• Where can I find help with Emacs?