It was a system of colonization used by the empire of Spain.
When the Spanish conquered the natives in the Caribbean, South America, and later the Philippines, these natives were required to pay tribute or perform work for Spanish landowners. In return the Native Americans would receive protection from other tribes, education, and Christianity.
This worked greatly in the favor of the Spanish colonists who received the trusteeship and land (an encomendado), but was little better than involuntary servitude for many native tribes.
(see related link)