Opening a TCP connection involves exchanging three packets. These packets are SYN, SYN+ACK, and ACK. They are called this way because the TCP header has SYN (for synchronize) and ACK (for acknowledge) bits. The first packet as the SYN bit set (equal to one), the second packet has both these bits set, the third packet has only the ACK bit set.
This exchange is used to establish initial sequence numbers of the connection. After the three-way handshake is complete, connection is established