If IP addresses are for finding the specific computer on a network you're wanting to talk to, Ports are for finding the specific application you want to talk to on that computer. So kinda like a phone extension. When an application "opens" a port, they're just telling the OS "hey, if any packets come in on this port, send the data my way, I'll know what to do with it".

A firewall is a special program the OS uses to control access to its ports. It says what programs are allowed to access what ports, effectively controlling the ability for all apps to access the network.

The only other thing to know is that the first 1024 port values are usually heavily controlled by the OS because there are specific protocols that are traditionally used on those specific ports, so you usually don't want just any application claiming one of those ports willy-nilly.

Oh, and you may have had to deal with "port forwarding" on your router. This is because, if some computer outside your network sends a packet to your router targeting a specific port number, the router doesn't know which computer it should go to. So by default, it just ignores it (which is usually the safest thing to do). Port forwarding tells your router, "if any packets come in on this port, send them to the computer at this IP, they'll know what to do with it."