Read receipts notify the senders of iMessages when the recipient reads the message. This is either great or evil depending on your perspective. My colleague and his wife both have read receipts turned on on their phones. That way if a timely SMS doesn't get read right away, they can call each other instead. Other people prefer that the senders of messages not get notified. That way there is no pressure for an immediate reply. Whichever camp you fall in, you'll need to know how to to turn read receipts on or off.