“Haint” (or “hant”) is most likely related to the word “haunt.” In Gullah folklore, a haint is a frustrated spirit caught between life and death that looks for a place to haunt. In other words, they’re pretty much the same as restless or wandering ghosts from many other cultures.
There are some distinctive features, however. For one thing, haints are unable to cross water. This gives rise to perhaps the most notable feature of haint-lore: painting one’s shutters, window frames, door sills, etc., blue in hopes of tricking the haint into avoiding the house, thinking it is prevented from getting in.
This practice is evident, for example, on St. Simons Island, and the color used is darker than the traditional “haint blue” one sees further up the coast. See, however, this post that suggests what is traditionally called “haint blue” is not the ghost-repelling color of Gullah folklore! (The blue window pictured above is from St. Simons.) The practice is also slightly different. In South Carolina, the idea seems to be that either (1) the blue color makes the haint fly up through a porch ceiling thinking it is ascending to the sky (and thus leaving the house alone), or (2) that there is something about the color blue that is itself repulsive to haints.
In any event, driving away haints by means of the color blue is a practice first brought to North America from Angola in the 1700s. “Haint blue” (whatever its authentic hue) is often seen in areas that were strongly influenced by African folklore, including coastal Georgia, coastal South Carolina, New Orleans, and the Caribbean.