A mystic society is a Carnival social organization, similar to a krewe in New Orleans, that presents parades and/or balls for the enjoyment of its members, guests, and the public.  The term came to be used in this context in Mobile, Alabama. Mystic society membership is secret. The societies have been based in class, economic and racial groups. Mobile's mystic societies build colorful Carnival floats and create costumes around each year's themes.

During the Carnival season, the mystic societies parade in costume on their floats throughout downtown Mobile. Masked society members toss small gifts, known as throws, to the parade spectators.  The throws can take the form of trinkets, candy, cookies, peanuts, panties, artificial roses, stuffed animals, doubloons, cups, hats, can coolers, Frisbees, medallion necklaces, bead necklaces of every variety, and Moon Pies.

Mystic societies in Mobile also give formal masquerade balls, known as bal masqués, which are almost

always invitation only and are oriented to adults.  Attendance at a ball requires a strict dresscode be followed that usually involves full-length evening gowns and white tie with tails for invited guests, and masked costumes for society members. The bal masqués feature dramatic entertainment, music, dancing, food, and drinks. Bal masqués are usually based on a theme which is carried out through scenery, decorations, costumes, and sometimes a tableau vivant.  Much of the memorabilia from these occasions can be viewed at the Mobile Carnival Museum.
For more at Wikipedia:  http://en.wikipedia.org/wiki/Mystic_society