In games like World of Warcraft, gamers can virtually "farm gold", which essentially means playing in a tedious and repetitive way to score some in-game currency--it might mean fighting the same bad guy over and over again, or completing a small task a few thousand times. Players (or, in this case, slavedrivers) actually sell that virtual gold for real-life currency, usually to people in developed countries who want a better character but don't feel like arduously spending days leveling up, and would rather just pay for the stats. Apparently, the guards at Liu's prison realized that they could make even more money forcing prisoners to farm gold than by forcing them to perform manual labor. The 300 prisoners forced to farm gold could earn their guards up to $1,000 per day, not that the prisoners ever saw a single yuan of it.