A farmhand is someone who's employed by a farmer to work on a farm. Farmhands might help harvest vegetables, feed animals, or drive a tractor.
If you get a summer job on a farm, you can call yourself a farmhand. While a farmer owns or manages a farm, a farmhand is hired to take on extra jobs, sometimes either part time, temporarily, or seasonally. The word hand gained the meaning "person who does something with his hands" in the 16th century, and often meant "hired workman" or "ship's crew member." Farmhand, therefore, means "hired worker on a farm."