Many simple commands, such as those invoked with a single key or with M-x command-name RET, can be repeated by invoking them with a numeric argument that serves as a repeat count (see Numeric Arguments). However, if the command you want to repeat prompts for input, or uses a numeric argument in another way, that method won’t work.
The command C-x z (repeat) provides another way to repeat
an Emacs command many times. This command repeats the previous Emacs
command, whatever that was. Repeating a command uses the same arguments
that were used before; it does not read new arguments each time.
To repeat the command more than once, type additional z’s: each z repeats the command one more time. Repetition ends when you type a character other than z or press a mouse button.
For example, suppose you type C-u 2 0 C-d to delete 20 characters. You can repeat that command (including its argument) three additional times, to delete a total of 80 characters, by typing C-x z z z. The first C-x z repeats the command once, and each subsequent z repeats it once again.
You can also activate repeat-mode which allows repeating
commands bound to sequences of two or more keys by typing a single
character. For example, after typing C-x u (undo,
see Undo) to undo the most recent edits, you can undo many more
edits by typing u u u…. Similarly, type C-x o o o… instead of C-x o C-x o C-x o… to switch
to the window several windows away. This works by entering a
transient repeating mode after you type the full key sequence that
invokes the command; the single-key shortcuts are shown in the echo
area.
Only some commands support repetition in repeat-mode; type
M-x describe-repeat-maps RET to see which ones.
The single-character shortcuts enabled by the transient repeating mode
do not need to be identical: for example, after typing C-x {, either { or } or ^ or v, or any series
that mixes these characters in any order, will resize the selected
window in respective ways. Similarly, after M-g n or
M-g p, typing any sequence of n and/or p in any mix
will repeat next-error and previous-error to navigate in
a *compilation* or *grep* buffer (see Compilation Mode).
Typing any key other than those defined to repeat the previous command
exits the transient repeating mode, and then the key you typed is
executed normally. You can also define a key which will exit the
transient repeating mode without executing the key which caused
the exit. To this end, customize the user option
repeat-exit-key to name a key; one natural value is RET.
Finally, it’s possible to break the repetition chain automatically
after some amount of idle time: customize the user option
repeat-exit-timeout to specify the idle time in seconds after
which this transient repetition mode will be turned off automatically.