Video narration
Why do two devices need three messages before they can start sending data? Think of a "Can you hear me?" check on a call: each reply tells you the last message got through. TCP makes that check before data begins. First, the client sends SYN: a request to connect, carrying its starting sequence number. It's the first "Can you hear me?" The server replies with SYN-ACK: "I got your request," plus its own starting sequence number. Each side has now sent its starting number, and the first message has been acknowledged. But the agreement still isn't complete. The first two messages don't prove the reply got back. The client sends ACK, meaning it received the server's reply. Now each side knows the other's opening message arrived. Like on your call, three messages confirm both sides' opening messages arrived; now TCP is ready for data.