I install lighttpd in one server using port 81.
The problem is that the port is closed, and I don't know how to open the port.
The 4th Coloumn is the IP address that have something listening on them. With 127.0.0.1 being the loopback interface. You might also see 0.0.0.0:81 for all interfaces instead.
Which if under the INPUT chain mean accept all incoming connections into the loopback interface. Also, notice the first two columns. This is how many packets / bytes are matching that rule, so you should see the counters changing with DROP or ACCEPT rules accordingly. It is a way to see if the rules are 'catching' the packets.
The above assumes you have ssh access or something similar and are on the machine itself. That is the best place to start, eliminates and complexity of the router with its possible NAT. You can do NAT with iptables, but not sure if you can do it with the loopback interface, and that would be an odd thing to do if you can.
Define "closed". If
How are you trying to access it? i.e. what address are you entering in the address bar? are you use