Most commands in GNU Emacs are entered in a special buffer called the minibuffer. Usually, it appears at the bottom of a frame (Read and learn more about the minibuffer here and here).
Sometimes, when using the minibuffer, you may want
to enter control characters. An example of a
use-case where you may want to use control
characters is when you have configured your editor
to visually see linefeed characters—“^M”—and
you want to use a facility like GNU Emacs’ isearch
to manually replace that character with something
else. To do so, use the in-built function:
(quoted-insert ARG) which is normally bound to
“C-q”. This reads the next input character and
inserts it—a useful thing when inserting control
characters. Here’s an example how to insert a
newline control sequence:
For what’s it worth, you can use octal digits to specify a character code too! Note that while inputting characters, non-digit characters terminate the sequence.
In summary, the important thing to note is that
you can use
quoted-insert (usually bound to “C-q”)
to insert any control character anywhere you want,
and that includes the minibuffer.