In Hinduism, the Sudra caste is the fourth, lowest and darkest-skinned caste of human beings. Sudras are mainly servants.
Based on an interpretation of the sacred scripture known as the Rig Veda, the sudras are believed to be the feet of the parusha, or “cosmic man.” Scholars subscribing to the Aryan invasion theory of the Indian subcontinent say that this caste was probably created by the Aryans to account for the indigenous, darker skinned Dravidians.
Historically, sudras were forbidden to read the Veda, which no doubt helped to keep them in their lowly social position.
For more and recent challenges to the old caste system, see » Caste
¹ The Aryan invasion theory was once largely unquestioned but is now challenged by some who, the majority says, overlook important data. Like most matters where ethnicity and religion overlap, it is a heated, politically charged debate. See https://en.wikipedia.org/wiki/Indo-Aryan_migration_theory#Controversy