It's mostly used as a sort of archetypal descriptor term; a southern belle refers to a young, rich/upperclass southern white woman from the Antebellum ("pre-war", in reference to the American Civil War) period. It definitely shouldn't be used as a compliment, seeing as they'd be from families that owned (or were closely financially connected to) plantations that profited off of slave labor.
The archetype is unfortunately still romanticized in certain parts of the south, where you'll see occasional "Antebellum" or "plantation" themed parties that try to evoke the image of the historical parties held by plantation owners, with similar style of dress, a heavy emphasis on ""manners"", and typically held on real plantation estates. Of course, the ugly truth of it is that these families' wealth was gained from the forced labor of enslaved people, and all the preparation and entertainment for these parties would've been done by house slaves.
Scarlett O'Hara from Gone with the Wind is probably the most famous fictional example of this archetype.