If you use WhatsApp you might know already it shows status of message delivery but from now on it is also going to show you when someone reads your messages.
This is a privacy concern because not everyone wants others to know when they read their messages. At present time there is no option to opt-out of this but WhatsApp developers will surely add opt-out feature soon in future updates.
This is beneficial for those who wait hours but have no idea whether recipient read their message or not.
One check mark = Message is sent from your phone
Double check marks = Message is delivered to the recipient
Double check marks highlighted in blue color = Recipient has read your message