WhatsApp will mark messages that have been read with two blue flag checkmarks. This confirms the company on its own site. The new blue checkmarks have a different meaning than the already existing gray checkmarks.
WhatsApp explains the meaning of the new checkmarks on it’s faq page, after being validated by Softonic that had spotted checkmarks in the Android version of the app. The blue checkmarks mean that a recipient has actually read the message, but in doing so it is necessary that both phones run the version of the app that supports it.
The two gray checkmarks only mean that the phone of the recipient received the message while a single tick still indicates that the message has been sent. Users of WhatsApp now know for sure that a message has been read with the arrival of the blue marks. The function also works in group discussions, when all participants have read the message.
It is unknown how the function fits together, but presumably WhatsApp marks a message as read when the recipient has opened the app and watched the conversation in which the message is sent. On iOS, Android and BlackBerry 10 it’s possible to read messages without opening the app.