On the report-viewing page in Mailer app you can see that all recipients are distributed among the following categories:
Exceptions contains recipients which were excluded from the mailing list, because they either had unsubscribed from your mailings before, or they could not receive one of your previous messages due to a critical delivery error.
Read speaks for itself, it contains recipients about which Mailer "knows" that they have opened (and probably read) your message.
Unsubscribed contains the recipients who have clicked on the "Unsubscribe" link contained in this specific message.
Bounced is convenient to view the list of recipients to which your newsletter could not be delivered because of an error on their mail server.
All other recipients not included in any of the above listed categories are listed under "Unknown", because you literally have absolutely no information on whether your message was read (perhaps images are disabled in their email clients) or whether it was delivered at all (perhaps a delivery error occurred but no notification was sent to your Return-Path mail box).
The only thing which you know about such "unknown" recipients is that Mail did try to send your message to them and that they have not unsubscribed from your mailings (otherwise they would have landed in the corresponding report category).