The Greats are mortals who have moved into the Otherworld, meaning they are deceased, and had special gifts as mortals. A prime example is Amarie Treadeau, who was a Seer, and therefore had abilities to see into the future and communicate with the dead. The known Greats are Sulla the Prophet, Uncle Abner, Aunt Ivy, Aunt Delilah and Twyla Valentin. It is likely that Amma has joined them after her death.
The Greats were only ever really mentioned until Ethan Wate got to personally meet and grow to know them when he died as a sacrifice and crossed over. They were seen enjoying the food Amma had given them previously.