TCP Handshakes are to establish or terminate the connection between sender and receiver. You can read more in detail about TCP here. Three-Way handshake is to establish the connection with the server. Four-Way handshake is to finish the connection
4 Way Handshake
TCP stands for Transmission Control Protocol. Just like humans, Computers also require language to communicate with each other. TCP/IP is a common language which most of the modern computers share to communicate to each other.