M-x

as

C-h k M-x

M-x runs the command execute-extended-command
    which is an interactive built-in function.

Read function name, then read its arguments and call it.

(execute-extended-command PREFIXARG)

note: I think that the word 'which' should be replaced with the word 'that', in order to be grammatically correct, but far be it from me to judge the grammar of documentation in emacs functions...

M-x is an Emacs key combination. (If the keyboard has "Meta" key, this means meta + x, in modern PC keyboard maps this often means Esc followed by x.)

There you'll get a prompt "M-x", after which you can write all sorts of commands. Tab completion works so you can see all commands.

Some useful commands:

• M-x replace-string
• M-x replace-regexp
• M-x revert-buffer
• M-x undo
• M-x gnus
• M-x doctor
• M-x praise-be-unto-xemacs

Some commands can get numerical arguments: For that, type C-u argument M-x command. For example, C-u 10 M-x dissociated-press tells Dissociated Press to use 10 character overlap.

You can also make your own elisp functions be launchable this way: Just put (interactive) as the first command in the function.