Once you have seen a message and determined the appropriate action (if any) for it, you may want to mark the message as Acknowledged. This lets you (and any other administrators viewing the messages) know that someone is already dealing with this message.
The Acknowledged flag appears in the last column of the tabular display in the Message Reader's results (right) pane. No (the initial message state) indicates the message has not been acknowledged; Yes indicates the message has been acknowledged.