I need to configure an iptables in order to blocks everything except web traffic and the file transfer protocol. I searched for information on the Internet and found these two commands. I don't understand the keywords ESTABLISHED and RELATED and --state. Please give me an explanation. Thank you so much.
iptables -I FORWARD -p tcp -m multiport --dports 21,80,443 -j ACCEPT
iptables -I FORWARD -m state --state ESTABLISHED,RELATED -j ACCEPT