Apple's iMessage app provides message status updates that let you know when a message has been delivered. It also has a handy feature called Read Receipts that lets you know when someone has read the message.
This status update can be a blessing when you want someone to know automatically that you have viewed their time-sensitive response. Apple handles the status update so you don't have to do anything other than open the message. Just read the iMessage and the sender will see the "Read" timestamp in their timeline.
As useful as the Read Receipts feature may be, it also can cause angst both for the sender and recipient. You can imagine the following scenario -- a wife discovers she is pregnant and sends an excited message to her husband. She sees that he read the message an hour ago and is upset because he never responded. On the other end is the husband, who viewed the message right before an important meeting. He now is agonizing over how to respond while his boss is breathing over his back, knowing that his wife is aware of the fact that the message has been read.
If you wish to avoid such complicated message relationships, you can easily turn off read receipts, which hides the fact that you have read a message from the sender. The sender will still see the "delivered" status, so they can be assured the message arrive on your handset. They just won't know if or when you have read the message. Follow these steps to turn off Read Receipts:
Open the settings app in iOS
Scroll down to "Messages" and tap on it to view the Messages settings
Find "Read Receipts" and make sure it is toggled to the off position (in iOS 7, green is on and clear is off)
Resume your normal messaging behavior
Once this option is toggled off, other people will not be alerted when you read their message.