Bridge enables you to send a message from your mail reader or web mail provider (Outlook, Gmail) to a special email address called the List Address, that 'll then be broadcasted (sent) , on behalf of you, to your list subscribers.
It also allows you to set up a discussion list that works the same way - subscribers to your list will have the oppourtunity to send to the List Address, which will then be broadcasted (sent) to your list subscribers.
Discussion Lists work fairly similar to what you may be familiar with, but if you do have any questions about what exactly can be done, let us know.