Online and last seen tell you if your contacts are online, or the last time they were using WhatsApp.
Online means that contact has WhatsApp open in the foreground on their device and is connected to the Internet. However, it does not necessarily mean they have read your message.
Last seen refers to the last time the contact used WhatsApp. Through our privacy settings, you have the option to control who can see your last seen. Please note you cannot hide your online. Learn more about privacy settings on: Android | iPhone | Windows Phone | Nokia S40 | BlackBerry | BlackBerry 10
There are a few reasons why you may not be able to see a contact's last seen: