Explain Like I'm Five

There's something important missing from most of the other answers. There's a lot of different kinds of network and internet traffic. Web browsing, email, instant messaging, online video games...

By formal standard, certain port numbers are designated for certain functions. Web traffic happens on port 80. Incoming email is sent on port 143, outgoing email is sent on port 456 or 587. Something like Discord will have a specific port it uses for both sending and receiving messages. Word of Warcraft has certain ports its uses for telling the server when you cast a spell, and for the server to tell your client when you take damage.

So yes, ports are like PO boxes at a post office, but the analogy doesn't quite capture it. Port 80 is always web traffic, and this is important, since your web browser requests pages on port 80, just as a web server returns web pages on port 80. The web server probably has other ports on it, like FTP (ports 20 and 21) or SFPT (port 22). If you connect to a web server on port 80, that means you're asking for its webpages. If you connect on 20, 21, or 22, it means you're trying to transfer files to it.