The economy of the antebellum South was centered on plantations whose workforce was largely made up of slaves. These plantations mostly produced "staple" crops. These are crops that are grown in bulk to be sold. This sort of a system contrasts with one in which smaller amounts of many more crops, mostly meant for local consumption. In the South, for example, most plantations produced cotton rather than a variety of kinds of foods.
This was important for two reasons. First, it meant that the South never really industrialized. Southern capital was tied up in land and slaves and could not be used to create factories and technology. Second, it meant that the South was an unequal society. The owners of large plantations made up an aristocratic elite who saw themselves as superior to the rest of the society.
Thus, the economy and society of the South were shaped largely by the fact that the South depended on plantations growing staple crops for export.