This defines which file to load when a function is called, plus doc-string, plus it indicates whether the function is interactive or not.

Example:

    (autoload 'python-mode "python-mode" "Python Mode." t)

This checks for python-mode.elc or python-mode.el on the LoadPath.

This works well with AutoModeAlist:

    (add-to-list 'auto-mode-alist '("\\.py\\'" . python-mode))

The autoload will make sure that the correct source file is loaded when `python-mode' is requested, and the `auto-mode-alist' setting will make sure that `python-mode' is requested when a ##.py## file is loaded.