A U.S. Southern style biscuit is similar to a British style plain scone, but with 1.5 to 2x as much butter in the dough, creating a soft and tender baked good that has a rich buttery crumb. The addition of buttermilk gives it a slight tang as well. More salty savory buttery than sweet, it's most often served as part of a meal, not by itself.
There are actually numerous variations of a Southern biscuit. Ranging from the flakey buttery type to the fluffy pillowy kind to "Angel biscuits" that use yeast and baking powder to give it extra lift. Nathalie Dupree had a whole cookbook dedicated to the subject called Southern Biscuits. Highly recommended!