b2 is a PHP/MySQL blog/web journal package.
More and more corporate enviornments are using blogs as internal communication tools. Being able to post to a blog directly from Tasks makes it very easy to put up a quick post about something you are working on.
If you have it enabled, there is a small 'b2' button in the toolbar on the Edit Screen. Press that button and it will send the task to the b2tasks_post.php page. From there, you can make any edits you want and post it to your blog.