Iptable is the administration tool for IPv4 packet filtering and NAT. Iptables is used to set up, maintain, and inspect the tables of IPv4 packet filter rules in the Linux kernel. Several different tables may be defined.Each table contains a number of built-in chains and may also contain user-defined chains.Each chain is a list of rules which can match a set of packets. Each rule specifies what to do with a packet that matches. This is called a `target’,which may be a jump to a user-defined chain in the same table.

Full article here:
18 Examples to Learn Iptable Rules On CentOS (LinOxide)