What does "closed" mean in this context?

It means if you sail through it, Iran will launch drones and missiles at your ship.

I'd say the analogy is a closed door.

It's not that it's impossible to go through it, but you have to do something specific in order to do so beyond just trying to go through, or you're going to walk straight into getting a bloodey nose.

But yeah, these ships weren't anywhere near the strait.

The strait is considered closed when a country not afraid to use its military says ships can't cross.

The Iranian military has stated their intention to attack any ship passing through the strait without their permission.