From what I read at the Bitnami Forum, it is a specific installation of MAMP that allows you to easily load modules of specific PHP-based web tools, such as WordPress, Drupal, etc., on top of the base MAMP platform.
I was actually looking for this as I am a web dev noob, and wanted to be able to learn something like Drupal while working on WordPress.
you can easily install WordPress or Drupal on MAMP also. Just download the files and place them on the root of your web server folder which is usually "htdocs".
WordPress has an easy how-to tutorial on how to install WordPress on a local web server like MAMP.