Conversation Permissions allow you to determine which user groups and users are allowed to perform certain conversation related operations. This includes who can add a conversation message and message attachments. Usually this is the lower level users who are populating the forums with messages, but you may only want a select group to be able to add attachments. For the more administrative actions, Edit Conversation Permissions is the highest level permission, followed by more "moderator" style permissions, such as deleting messages, editing messages, flagging messages, and approving messages. The rating of messages is allowed for registered users so that they can upvote / downvote messages, but you may want that to be available to only a select group, so you could change that here.
You'll notice there is a permissions entity called "Message Author" in use here so that message authors can edit and delete their own messages. This is useful so users can manage their own content if they wish.