Let's say you want to talk to your friend. You have several protocols, Phone, Mail, Email, SMS, or maybe something stranger, like smoke signals or memes. Each protocol needs a different port.

Your friend doesn't answer phone calls or check VMs. The 'port' for phone calls is blocked. Nothing gets in through here.

Your friend only accepts email from certain addresses, like protonmail. This port is filtered. Only known things get in through here.

Your friend accepts any texts if they begin with the secret pass phrase. This port is open and filtered. Few unknown things get in through here.

Your friend accepts all postage. This port is open. Anything can get in through here. HTTP or HTTPS, your browser.

The VPN is sort of like an opaque tunnel you run from your house alongside public roads to some place you feel safe exiting, And then the usual steps in communicating. All of the communication has to go through the exit point back to the house.

If this kind of thing interests you, have a basic book for free. https://open.umn.edu/opentextbooks/textbooks/353