Farming is used to grow crops of Cotton, Flax, Wheat and Food (Carrot, Corn, Potato, Pumpkin, Turnip).

Farms are designated under Design>Farms. You must use flat dirt or grass blocks for farming. Farmers will need a Hoe as well as Seeds to plant.

After a seed has been planted in a designated block it will grow until mature. If left too long after mature, plants will wilt and disappear.

Farmers can speed up crop growth by tending to them, but this isn't required to for plants to fully mature. Considering this it maybe be more efficient to have your Farmers doing other things rather than tending to crops, like simply planting & harvesting more crops.