What is TCP in networking?

TCP = Transmission Control Protocol. It is the other half of the Internet Protocol. Every network has its own way of communicating. The most commonly used is the IP/TCP stack. Going back in time there were the NetBEUI NetBIOS, IPX/SPX and others that were pretty much proprietary--i.e. they were made by companies that made network software and operating systems such as Novell and its NetWare software.. TCP/IP has replaced them all as the standard. It is in the "Transport Layer" of the OSI model. If you're not into networking hardware and software engineering, you have no need to know the OSI model.