What is Iptables ?

iptables - Administration tool for IPv4 packet filtering and NAT

Iptables  is  used  to set up, maintain, and inspect the tables of IP 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.

A  firewall  rule  specifies  criteria for a packet, and a target.  If the packet does not match, the next rule in the chain is the examined; if it does match, then the next rule is specified by the value of the target, which can be the name of a user-defined chain or one of the  special  values  ACCEPT,DROP, QUEUE, or REJECT