what is the NTH?

hello

I studied wiki and documents for this service
this service is in tabs : ip > firewall > mangle
and i am not understanding any content
for example this line :

How it works in v3.0

Every rule has its own counter. When rule receives packet counter for current rule is increased by one. If counter matches value of ‘every’ packet will be matched and counter will be set to zero.

If passthrough is not set then packets will be marked as follows:

first rule nth=2,1 rule will match every first packet of 2, hence, 50% of all the traffic that is matched by the rules
second rule if passthrough=no will match ONLY 25% of traffic because in 3.0 you need only one rule to catch traffic not like 2.9

plz help me
Tell a practical example for this service

For e.g. NTH could be used for rr Load Balancing of WAN’s.

Please look here

http://wiki.mikrotik.com/wiki/NTH_load_balancing_with_masquerade

if you like to drop one packet, every 5 packets, you would use nth.
if you like to send every other packet to different gateway, you would also use nth.