The discussion board is a place where you and your users can post topics you wish to communicate about in an open forum. Whether this forum is public or not is up to you, you can change this in your privacy settings area. If you do not wish to use the discussion board widget, you can turn it off in your widget settings. See widgets for more info.
Users can comment on the posts; the number to the left of the topic will show the comment count.
If you click 'view all' or navigate to the discussion board page, you will see all the open topics listed there.
When you create or edit new discussion board post, you have the option to keep it as 'sticky', meaning it will always display at the top of the topic stack. You can also restrict the topic from further comments (this can be good for older posts).
The discussion board is a good forum for posing questions to your members, or for open discussion about your team. Your team members will not be notified about new discussion posts, but they can follow posts that interest them by opting to 'watch topic' on the topic page. This will trigger an email to be sent when there are new comments on this topic.