TCP is a 16-bit port number used to identify a specific software program on the server hardware.
Port numbers starts from 0 to 65535 and in this 0 to 1023 are reserved for well known services.
Some import port numbers:
21 - FTP (File Transfer Protocol)
23 - Telnet
25 - SMTP (Simple Mail Transfer Protocol)
37 - Time
80 - HTTP (Hyper Text Transfer Protocol)
110 - POP2 (Post Office Protocol 2)
443 - HTTPS (Hypertext Transfer Protocol Secure)