Explain like im 16 (or something)

One thing that people dont get is what firewall actually does - It blocks

• no open ports - cant get in, solid wall everywhere
• open ports - as if you put doors somewhere in the wall, depending on doors quality they can get in easily or not at all

That minecraft is a good example. You open port to it, people can join it, but depending on how you setup your minecraft server, if it has plugins that require registration and password to move around, well that either stop people from griefing or not.