A port is a numbered doorway a computer uses for a particular kind of connection. Think of an office building with many doors, each leading to a different service inside.
When a browser contacts a website, the address may use a standard port without showing the number. Developers sometimes use another port while testing a site on their own computer.
Most visitors never need to choose a port. If someone shares a local testing address, it may include a colon and a number to point the browser to the right doorway.