auto-fill-mode is an interactive compiled Lisp function in `simple.el'.

(auto-fill-mode &optional ARG)

Toggle automatic line breaking (Auto Fill mode).
With a prefix argument ARG, enable Auto Fill mode if ARG is
positive, and disable it otherwise. If called from Lisp, enable
the mode if ARG is omitted or nil.

When Auto Fill mode is enabled, inserting a space at a column
beyond `current-fill-column' automatically breaks the line at a
previous space.

When `auto-fill-mode' is on, the `auto-fill-function' variable is

The value of `normal-auto-fill-function' specifies the function to use
for `auto-fill-function' when turning Auto Fill mode on.