The Emacs commands that are used often or that must be quick to type are bound to keys---short sequences of characters---for convenient use. Other Emacs commands that do not need to be brief are not bound to keys; to run them, you must refer to them by name.
A command name is, by convention, made up of one or more words, separated by hyphens; for example, auto-fill-mode or manual-entry . The use of English words makes the command name easier to remember than a key made up of obscure characters, even though it is more characters to type.
The way to run a command by name is to start with M-x, type the command name, and finish it with RET. M-x uses the minibuffer to read the command name. RET exits the minibuffer and runs the command. The string `M-x' appears at the beginning of the minibuffer as a prompt to remind you to enter the name of a command to be run. See Minibuffer, for full information on the features of the minibuffer.
You can use completion to enter the command name. For example, the command forward-char can be invoked by name by typing
M-x forward-char RET
or
M-x forw TAB c RET
Note that forward-char is the same command that you invoke with the key C-f. You can run any Emacs command by name using M-x, whether or not any keys are bound to it.
If you type C-g while the command name is being read, you cancel the M-x command and get out of the minibuffer, ending up at top level.
To pass a numeric argument to the command you are invoking with M-x, specify the numeric argument before the M-x. M-x passes the argument along to the command it runs. The argument value appears in the prompt while the command name is being read.
Normally, when describing a command that is run by name, we omit the RET that is needed to terminate the name. Thus we might speak of M-x auto-fill-mode rather than M-x auto-fill-mode RET. We mention the RET only when there is a need to emphasize its presence, such as when we show the command together with following arguments.
M-x works by running the command execute-extended-command , which is responsible for reading the name of another command and invoking it.