What is a handshake in networking?

There are two different handshakes, I'll go with the basic TCP one though.

Say you have two different people.

Sally and Bob

Sally is the first host, and bob is the second host.

Sally sends bob a synchronized message (SYN) which bob receives

Bob then replies that he acknowledged it, synchronized-acknowledged (SYN-ACK)

Sally then replies that she acknowledged the acknowledgement and bob doesn't reply.