Emacs Lisp: What is the Function fn?

it's a doc string convention, to specify the form of a function, as the last line in the doc string.

For example,

(autoload 'auto-revert-mode "autorevert" "\
Toggle reverting buffer when the file changes (Auto Revert mode).
With a prefix argument ARG, enable Auto Revert mode if ARG is
positive, and disable it otherwise.  If called from Lisp, enable
the mode if ARG is omitted or nil.

Auto Revert mode is a minor mode that affects only the current
buffer.  When enabled, it reverts the buffer when the file on
disk changes.

Use `global-auto-revert-mode' to automatically revert all buffers.
Use `auto-revert-tail-mode' if you know that the file will only grow
without being changed in the part that is already in the buffer.

\(fn &optional ARG)" t nil)

Functions such as describe-function will use that line to generate the function's form to display the doc string.

This is sometimes necessary because some functions, such as macro, the usage isn't the same as the defmacro parameters.