Your textbook defines a southern planter as an individual who owned at least slaves.

A southern planter referred to an individual who owned at least 20 slaves during the Plantation Era (Antebellum era) in the (Southern) United States from late 18th century until 1861, which was the start of the American Civil War. 

Planters referred to the people who owned plantations during the Antebellum Era. Most of the planters comprised the elite class of the South and differed from the farmers in that they were more like businessmen and did not work alongside their slaves on the fields.