This target allows to alter the MSS value of TCP SYN packets, to control the maximum size for that connection (usually limiting it to your outgoing interface's MTU minus 40). Of course, it can only be used in conjunction with -p tcp.
This target is used to overcome criminally braindead ISPs or servers which block ICMP Fragmentation Needed packets. The symptoms of this problem are that everything works fine from your Linux firewall/router, but machines behind it can never exchange large packets:
    1)  Web browsers connect, then hang with no data received.
    2)  Small mail works fine, but large emails hang.
    3)  ssh works fine, but scp hangs after initial handshaking.
Workaround: activate this option and add a rule to your firewall configuration like:
 iptables -A FORWARD -p tcp --tcp-flags SYN,RST SYN \
             -j TCPMSS --clamp-mss-to-pmtu

--set-mss value
    Explicitly set MSS option to specified value.

--clamp-mss-to-pmtu
    Automatically clamp MSS value to (path_MTU - 40).

These options are mutually exclusive.