In the postal system, the address on an envelope indicates a physical location, such as a residence or office building. But there may be more than one person at the location; thus you have to further quantify the recipient by putting a person or company name on the envelope.
In the phone system, one phone number may represent an entire company, in which case you need a person's extension number in order to reach that individual directly. Or, when you call a home, you have to say, “May I please speak to ...” before talking to the person directly.
IP networking provides the concept of addressing. An IP address represents a particular computer, but no more. In order to reach the mail service on a system, or the FTP or WWW service on a system, you must have some way to further specify which service you want. In the Internet Protocol suite, this is done with port numbers, which represent the services, much like an extension number used with a phone number.
Port numbers are 16-bit integers. Unix and Unix-like systems reserve ports below 1024 for “well known” services, such as SMTP, FTP, and HTTP. Numbers 1024 and above may be used by any application, although there is no promise made that a particular port number is always available.