Can someone demystify computer Ports for me? Please? Blocking, unblocking, opening, allowing, VPNs and their effect, what ports are and what they do, step by step, when you have to interact with them?

It's the one thing when I'm configuring things that makes me wince because I know it will give me the business, and I know it shouldn't, but it does, every time. I have no real idea what I'm doing, what it is, how it works, so of course I'm blindly following instructions like a monkey at a typewriter.

Please guide me into enlightenment.