The Window field in the TCP header provides a flow control mechanism for the connection.
The purpose of the Window field is to ensure that the sending computer doesn't send too much data too quickly, which could lead to a situation in which data is lost because the receiving computer can't process incoming segments as quickly as the sending computer can transmit them.
The flow control method used by TCP is called the sliding window method. The receiving computer uses the Window field (also known as the buffer size field) to define a window of sequence numbers beyond the last acknowledged sequence number that the sending computer is authorized to transmit.
The sending computer cannot transmit beyond that window until it receives the next acknowledgment.