iptables mean ?
 

/sbin/iptables -t mangle -A PREROUTING -p tcp \
-d n.n.n.n/32 \
--dport 21 -j MARK --set-mark 21
/sbin/iptables -t mangle -A PREROUTING -p tcp \
-d n.n.n.n/32 \
--dport 10000:20000 -j MARK --set-mark 21

There is iptables command when i want use FTp in NAT Router implementation. But I dont understand about "Mark" ?

'MARK' is just the name of a custom chain that you or someone created. Anything matching that criteria will be sent to the MARK chain for further handling.

I'd recommend naming things better, or renaming it once you understand what that chain does.