It litterally means conqueror. They were the leaders of the epeditions that took the new world for Spain. They were a quasilegal group. With men being given the right to conquer in the name of the king and govern what they took. They also split the loot between themselfs, their men, and the king. Although "their men" usually mean't their brothes/cousins and buddies they took with them to lead the expeditions. Also most of them were from minor Castillean nobility (or in Pizarro's case the illegitimate son of one) looking to make their fortunes in the new world.