What "open" and "closed" mean
This tool asks our server to open a TCP connection back to your public IP on the port you choose. If the connection succeeds, the port is open - traffic from the internet can reach a device behind your router on that port. If it times out or is refused, the port is closed from the outside.
Confirming port forwarding
The most common reason to run this is to verify a port forwarding rule. After forwarding a port on your router to a device's local IP, check it here. A closed result usually means the rule is missing or wrong, the service isn't running, a firewall is blocking it, or your ISP uses CGNAT (which makes inbound forwarding impossible without extra steps).
A word on security
An open port is an entry point, so only forward what you need and keep the service behind it patched. Read is port forwarding safe? for the tradeoffs, and use the common ports reference to see what a given port is typically used for.