|Port States NMAP will categorize ports as being in one of the following states:|
Open – The port is accepting TCP connections and UDP packets. This means that an application is running that is using this port.
Closed – The port responds to NMAP probe requests but no application is using this port
Filtered – The port state cannot be determined because packet filters prevent NMAP probes from reaching the port
Unfiltered – The port is accessible but NMAP cannot determine if it is open or closed
Open | Filtered – NMAP cannot determine if the port is open or filtered
Closed | Filtered – NMAP cannot determine if the port is closed or filtered.
Here is a great way to explain it.
Use the analogy of the cops coming to your place and looking for you. Having a filtered port is like them knocking on the door, and you saying "I'm not here". Having a closed port is like them knocking on the door, and no one answering. With a filtered port, you know there is an active system behind that port.