Lol. A serious answer is if you want a heavily populated server, Asura is #1 but it's known to be a merc server, where anything and everything is merc'd. Odin and Bahamut are the next most populated servers with NA and JP players fluctuate depending on the time of the day.
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It means you pay people ("mercenaries") to do the work of killing the monster(s) for you while you stand there with your thumb in your mouth like a wee little baby.