emacs-speak for "press the 'Meta' key and the 'x' key at the same time.


C-h k M-x
will tell you in emacs:

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...