Cornrows are a way of wearing hair: many small, tight plaits (braids) made in straight lines along the head, close to the scalp. The lines look a little like rows of maize in a field, which is how the name arose. The style is tied to African and African-diaspora hair traditions; it is a real cultural hairstyle, not a joke about vegetables. English almost always uses the plural, because you have many rows, not one. Doing cornrows takes time and skill. One row can be called a cornrow, but people usually talk about the whole style: hair in cornrows.