You pay for it with crowns. It's actually not difficult to test, just click on barb and you'll see icon in the bottom left, you click this icon and it tells you how to make it work.
For 35 crowns, you can increase a barbarian village's wood, clay, or iron production by 100%. In addition, the village's warehouse gets 50% storage capacity. But it lasts for only 3 days.
yh lol, there is a scrub that paid for 3 villages close to one of my m8's and he was quick to take the 700/700/700 resources^^
it's suicide and epic stuipidly to boost a barb close to a micro farmer.