I assume you mean in Africa?
In that case, most people were enslaved as a result of wars. The losing party in wars and other conflicts would often have to surrender large numbers of their people to the winner and they would be part of a subservient class that would eventually be incorporated into the victor's culture. It could also be a raid ( by Europeans or Africans) or punishment for a crime. People could be marched from the deep interior of Africa to slave trading centers on the coast of africa in order to be sold to European slave traders. Ultimately though they were regarded as slaves the whole time, but thei nature of the slavery they experienced change once they were sold to Europeans whose form of slavery was heritable and much more harsh in terms of working conditions