I learned that
M-x invokes command
What command does
I looked through the reference but failed to find the answer.
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In the default configuration, it is not a complete key sequence: it is a prefix key. It is an incomplete specification: it has to be followed by another key (or more than one, if the next key is also a prefix key) before it gets you to a complete key sequence (i.e. one that is bound to a command).
C-x C-f is bound to the command
find-file (by default),
C-x f is bound to
C-x n is bound to another prefix key, which could be completed with e.g. another
C-x n n is bound to the command
narrow-to-region. See the section on keymaps in the Emacs Lisp manual for details.