Last Seen and online tell you the last time your contacts were using WhatsApp, or if they're online.
If a contact is online, they have WhatsApp open in the foreground on their device and are connected to the Internet. However, it doesn't necessarily mean the contact has 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 can't hide your online. Learn more about privacy settings on: Android | iPhone | Windows Phone
There are a few reasons why you might not be able to see a contact's Last Seen: