The autoload module is a utility module. It allows other modules to leverage PHP 5's class autoloading capabilities in a unified fashion. It does, naturally, require PHP 5.1.2 or later.
You do not need this module unless you are developing a module that makes use of classes or you are installing a module that depends on this one.
Drupal 7's integrated self-learning code registry is far better than this module anyway, so this module will never be ported to Drupal 7. Drupal 6 is the only version that will be supported, but the code conventions used by this module will make moving to Drupal 7 easier for class-using modules.
See the included README.txt file for information on how to use this module in your own modules.