This is used to send back an error packet in response to the matched packet: otherwise it is equivalent to DROP so it is a terminating TARGET, ending rule traversal. This target is only valid in the INPUT, FORWARD and OUTPUT chains, and user-defined chains which are only called from those chains. The following option controls the nature of the error packet returned:

--reject-with type
    The type given can be
     icmp6-no-route
     no-route
     icmp6-adm-prohibited
     adm-prohibited
     icmp6-addr-unreachable
     addr-unreach
     icmp6-port-unreachable
     port-unreach
    which return the appropriate IPv6-ICMP error message (port-unreach is the default). Finally, the option tcp-reset can be used on rules which only match the TCP protocol: this causes a TCP RST packet to be sent back. This is mainly useful for blocking ident (113/tcp) probes which frequently occur when sending mail to broken mail hosts (which won't accept your mail otherwise).