A Heuermann was a tenant farmer in Northern Germany in the 19th century. Due to high taxation, many farm owners started to lease out small parcels of their land. Farmers who were unable to own land, could lease these small farm blocks, and live and work on the land as if it was their own. However, the land available for lease was usually the least productive land, and often could barely raise enough food for four people, not enough to properly feed a family, or sell produce to make living. As well, there were often arduous committments to the owner of the farm as a part of the lease agreement. Therefore, many emigrated to America, where they were able to own their own land, and not only feed their family, but make a successful living. The Heuerling System died out in the 1960's.
For further information, see http://www.honkomp.de/damme-auswanderung/heuerman.htm