Once activated/configured the plugin will bridge together WordPress accounts with phpBB accounts. This allows your users to register on your WordPress site and automatically be registered/logged into the phpBB forum. If you have existing phpBB members, they can link their two accounts by logging into phpBB from within WordPress once.
The plugin has been tested to work on WordPress 2.7.x and phpBB 3.0.x.
I remember when I published WordPress Weekly episode 36 where I interviewed Douglas Bell regarding phpBB and discussed the differences between the software and how important it would be to have a bridge between the two projects. The comments lit up with interest regarding a working bridge between the two systems. It looks like Brad is well on his way accomplishing this feat.