"last seen" does it mean that the person opened MY chat window with him, saw the conversation we had and then went offline?
or "last seen" only means that the person was using WhatsApp and not particularly my chat window with them ?
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 and is connected to the internet. However, it does not necessarily mean they have read your chat. last seen refers to the last time the contact used WhatsApp.