The title is really bad, but I was able to find better. I have installed Debian on VMware, and set iptables to block any traffic except port 80 (inbound and outbound). When I run a nmap scan it shows that this port is filtered. Should it be this why or not?
When nmap scans a TCP port (e.g. TCP/80 for HTTP traffic) a filtered response means that nmap did not get any response to the packet it sent. The other options for TCP ports are "closed" which means that in response to the SYN packet nmap sent, the host sent a RST packet (essentially indicating that there is no service listening on that port) or "open" which means that nmap got an ACK packet back from the port (generally indicating that there is a service on that port which is happy to receive connections).
If you can connect to the webserver on port 80 on the machine in question from another system then that indicates that port 80 is open and receiving connections, so if you carry out an nmap scan from the same machine you browse from, you should have received an open response as the server is there and available for connection.
Filtered is a loose term when it comes to nmap, in most cases it refers to a firewall guarding it. In some cases these protections may be circumvented. In your case I think it's safe to say that if your config is right, you're safe (on ports other than 80)