Chapter 18 - Ports
It's really just a number between 0 and 65535, used to allow multiple services to be served by a single IP address / multiple simultaneous connections to be made by a single client.
Some of the numbers have "well known" meanings, like everyone agrees 80 is for HTTP traffic, 443 is for HTTPS, 631 for IPP, 22 for SSH, 23 for Telnet, 25565 for Minecraft, etc
Any service can be configured to communicate on any port, those are just the defaults when you do not specify
When a client makes a request to a server, it sets a random source port so that when the server replies that reply is tagged with the unique response port and can be identified
And to be clear, "port" is meant in the same sense as an airport or a sea port. It identifies a destination for traffic
Granted, in the analogy of an airport it's more like a terminal / gate number than the whole port. Or a specific dock at a sea port. But I think the consideration is that if your computer was a country, it could service traffic at multiple ports simultaneously, where that traffic comes from or goes to any other location
Of course, "simulaneous" is in the sense of whole streams. Because Ethernet is a serial interface, your PC only receives individual packets one at a time. Really, really fast
I guess if you consider the computer as the country, the TCP/IP ports as airports, that would make individual terminals as socket descriptors, and the gates would be threads