It was called "work," that is, putting Indians to work on Spanish farms, mines, etc. It was a means of extracting labor from them, while teaching them the rudiments of the Spanish language (and training "overseers" in the indigenous language), as well as other aspects of "modern" society, such as using metal "coins" for money (some Indian civilizations used cacao beans or other perishables).
The Spanish reinforced this process by sending missionaries to convert Indians to Christianity, and establishing missions These missions ran schools for both the formal and informal instruction of the Indians, particularly children.
These, and other attempts to integrate Indians into Spanish society gave them a "stake" in the system. While many died under various forms of mistreatment, more survived than in North American societies, which tried to expel them, instead of "civilizing" them. Put another way, while a few of the more fortunate North American Indians were sent to "Anglo" style boarding schools to satisfy English ideals, comparable schooling was much more widespread in South America. That was in large part because there were several large, competing Catholic orders there all working (in different ways) to civilize the native Americans.