When you send a text to someone that has Read Receipts enabled on their iPhone, iPad or Mac, the word "Read" will appear beneath your message when they view it, and will also include the time that it was viewed. This can be especially useful for individuals that are in a situation where it would be inappropriate to text back, but need to make sure the other person is aware that they viewed their message.
To turn on Read Receipts in iMessage on your iPhone or iPad, go to Settings > Messages > Enable Send Read Receipts.
You can also choose who gets to see when you’ve read their messages from within the app itself. To enable this for an individual, open any conversation and tap your contact’s name, then scroll down and enable Send Read Receipts for that person.
Send read receipts in Messages on Mac
Send read receipts for one conversation
In the Messages app on your Mac, select a conversation.
Click the Details button in the top-right corner, then select Send Read Receipts.
Send read receipts for all conversations
In the Messages app on your Mac, choose Messages > Preferences, then click iMessage.
In the Settings pane, select Send read receipts.