Imagine there’s a massive wall into a country. The only way to pass through is a road with guards working. It has 2 directions, incoming and outgoing.

The people working there have a set of rules that you have given them that they have to stick to. Eg. Only let a specific person in and turn everyone else around, let everyone in, turn everyone around, turn around a person with a specific ticket number (port numbers)

There’s some more concepts to this, but that’s the gist of it.