-Pn (formerly known as -P0 and -PN) instructs Nmap to skip the host discovery phase, and instead assume that all hosts are up.

there's either nothing there, or there's a firewall configured to drop all traffic directed to it.

The nmap result "filtered" implies that (if you know there is a host with that IP address) access to the port has been blocked by a firewall or similar, which is dropping the traffic.

It's possible that the host's firewall has rules that are denying access to the IP from which you're running the scan