What is a TCP window update?

Question

I'm making my own custom server software for a game in Java (the game and original server software were written with Java). There isn't any protocol documentation available, so I am having to read the packets with Wireshark.

While a client is connecting the server sends it the level file in Gzip format. At about 94 packets into sending the level, my server crashes the client with an ArrayIndexOutOfBoundsException. According to the capture file from the original server, it sends a TCP Window Update at about that point. What is a TCP Window Update, and how would I send one using a SocketChannel?