Wherefore David arose and went, he and his men, and slew of the Philistines two hundred men; and David brought their foreskins, and they gave them in full tale to the king, that he might be the king’s son in law. And Saul gave him Michal his daughter to wife.
So, David and his group went and defeated 200 Philistine warriors. They brought the foreskins as proof to the king, hoping to marry the king's daughter, Michal. Saul then gave Michal to David as his wife.