What is a cash crop?
Crops that generate cash income rather than food for example: coffee, cotton and sugar.
Many farmers could subsist on the food grown their farm. They can barter with other farmers for other types of local food. To generate cash to expand or buy manufactured products or services they however needed crops that had a market value in cash. Items that can be sold internationally like coffee, cotton, and sugar can be sold for cash.
If a farmer can sell his labor to a plantation owner who grows a cash crop then this is another way to generate income.