What is a port number?

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Answer:

Where port numbers are concerned, consider an analogy between the computer and the telephone. Port numbers and their associated network addresses work something like telephone numbers and associated extensions.
One can think of a network address - say, an IP address - like a telephone number. The IP address gives a unique number for reaching someone on the Internet, just as a telephone number allows you to reach a specific destination on the public telephone network.

(Note that telephone numbers have additional features that IP addresses do not have, such as area codes that provide some geographic information about a location.)

Many larger organizations set up their telephone networks to use phone extensions. These extensions are typically 2-5 digit numbers that identify an individual phone within the organization. But from the outside, all of these extensions are associated with a single telephone number (in the USA, often a 1-800 or 1-888 number).

A network port number functions similarly to a telephone extension. Taken together with a network address, a port number identifies both a computer and also a "channel" within that computer where network communication will take place. Just as different organizations may use the same extension numbers "inside" their primary phone number, different computers use the same set of port numbers.