When does imessage say read?

Ok this might seem like a stupid question but i just got an iphone and was wondering the thing only will say read to the other person after you actuallly open the message right? Not just like when you're on your phone and it knows you saw it because the message went across the top of your screen?

Its says read when people using iOS have read the message, and I believe They have opened it. Some people don't have it turned on, (You can turn it on and off in Settings>Messaging>Turn on 'Send Read Receipts)