Messages that you have opened are marked as read. Conversations containing unread messages are shown in bold.
You can set preferences for whether or not to mark a message as read when you read the message in the Reading Pane. You can set your preference to mark a message that you review in the reading pane as read or only mark a message as read if you open it in the Content Pane (or in a separate window). These can be set in the Preferences > Mail tab.
You can change a message's status back from read to unread by right-clicking on the message and choosing Mark as Unread from the menu that appears. You can also mark a conversation as unread.
Marking a conversation as unread marks all messages within that conversation as unread.
Marking a single message as unread marks the conversation that contains the message unread.
You can use the same method to mark unread mail as read.