It's about unjust socioeconomic systems and our multifaceted complicity in them. The vehicle is a man's journey through the hell of chattel slavery, but the man as well as the institution are just a vehicle. The stars are all the different ways that unjust institutions maintain the power we give them. The film does both the journey and the theme justice with a minimum of false redemption.
The are many powerful images and actions that are specific to slavery, but characters responses to those events could be taken right out of the film and be at home in any American period, including our own. I hope that people see that and keep it in mind while watching.
Also, it's beautifully shot, scored and directed. A technical masterpiece at the least.