What is the match --mark field in Iptables
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in the below example 
 iptables -A FORWARD -i eth0 -p tcp --syn -m mark  --mark 0x3 -j REJECT --reject-with tcp-reset

what is the significance of -m mark --mark field. 

When i am trying to add this rule i am getting following error. 

iptables: Protocol wrong type for socket ?

Thanks,