You all know now that Facebook purchased WhatsApp in February for $19 billion.So,little modifications from Facebook were expected.The changes are starting to show now.Previously one white cheek meant you sent the message to WhatsApp servers successfully and two white cheeks meant that the person on the other end received it.But now WhatsApp included detailed ‘seen’ and ‘read’ information for messages you sent to either individuals or groups.It was a update from their server.So you don’t have to update the app. 
Source