A window (WIN) is the amount of data that the destination is able to receive. Although the window size is determined by the destination, the problem also extends to the source. If the source is on a fast network and the destination is on a slow network, then the source could literally jam up the network with data up to the window size. Since the network would not be able to transfer such large amounts of data, the network would get congested and the data that the network is not able to deliver would be thrown away. Routers enter IP datagrams into buffer memory, but even buffer memory is limited.
Data loss is always bad and our goal is to avoid it whenever possible. That is why we also define a window on the source side. This window tries to specify how much unconfirmed data the source can send before the network gets congested. The source-side window is called the congestion window (or CWND for short). The source gradually increases the CWND, but cannot increase it...