The simplest description, and the one that stuck with me, is telephone extensions. You have one general phone number (your computer's IP), but to reach a certain program inside the computer you have to dial a certain extension (port number) as the program is only listening on that particular extension. Some of those extensions are predefined so you don't need the know them, they are programmed into the sender software (web traffic over 443 (https) or 80 (http) for example, others you need to know the extension to be able to communicate (a friend setting up a Minecraft server for example). A firewall can block certain types of traffic by not allowing traffic to certain extensions (you can't communicate with the torrent client if those extensions are blocked), port forwarding works by taking traffic for a certain extension and sending it out over another, ...