As you may recall from earlier chapters in this book, a port is like a channel that's used for a particular sort of Internet data by one application at a time. When a program or service is listening on a given port —meaning it accepts incoming connections —that port is said to be open. Your Mac may use one port to send out data that's then received by a different port on another computer and vice versa. Most Internet traffic uses TCP, but some of it uses UDP, and a given port number can be used simultaneously by both protocols on a given computer.
The total number of ports available is 65,536 (numbered from 0 to 65535). Of these, the first 1,024 (numbered from 0 to 1023) are referred to as the Well-Known Ports, and for most of them, their official uses are registered with the Internet Assigned Numbers Authority (IANA) — meaning they're seldom used for anything other than a single designated purpose. Ports ranging from 1024 to 49151 are called Registered Ports. A large number of these also have official usage designations per the IANA, although there are still thousands that can be freely used by any application. Finally, the ports in the range 49152 to 65535 are referred to as dynamic or private. They can be used but aren't eligible for registration with the IANA.