This is a module that lets Drupal handle user- and session-management for phpBB, a popular forum / bulletin board system. In practice it takes things from the Drupal database and updates the phpBB database each time a user logs in. It also provides a block that lists recent topics from the forum. This module requires mod_rewrite / clean URLs and is not recommended for beginners.
The installation instructions can be found here. If you have questions or other concerns you should discuss those over at http://groups.drupal.org/phpbb, don't forget to say what versions of phpBB and Drupal you are using.
Note that the modules for phpBB 2.x and 3.x are rather different, but the matrix of workable combinations is as follows:
Drupal 6.x :: phpBB 3.x (planned)
Drupal 5.x :: phpBB 2.x and phpBB 3.x
Drupal 4.x :: phpBB 2.x