From whatsapp themselves:
The timestamps "online" and "last seen at..." tell you if your contacts are online, or the last time they were connected to WhatsApp.
"online" means that contact has WhatsApp open and is connected to the internet. However, it does not necessarily mean they have read your chat.
"last seen at..." refers to the time the contact left WhatsApp. You can also think of this as "went offline at..."
Note: If both the "last seen at..." and "online" timestamps do not display for one of your contacts, you may have been blocked. Learn more here.