In the Bible, David gave Saul a dowry of 200 foreskins of the Philistines for Michal. Saul had requested this as a bride price, hoping that David would be killed in the process of obtaining them. However, David was successful and presented the foreskins to Saul as proof of his accomplishment. This story is found in the Old Testament book of 1 Samuel, chapter 18, verses 25-27.
In Genesis chapter 17, God told Abraham to circumcise every male member of his family. The people in Israel came from Abraham’s family, so they did that. However, in most other nations (including Philistia), people did not follow that custom.
To circumcise means to cut a small piece of skin (called the foreskin) from the end of the male sex part.
Saul’s price for David to marry his daughter Michal, was that  David must kill 100 men from Philistia  (called Philistines). Then David would bring their foreskins as  evidence that he had carried out that task. However, Saul’s real plan was that the Philistines would kill David.
At this time, David commanded a group of 1000 soldiers (18:13). That was not enough men to fight a battle against Philistia’s army or to attack its main towns. So  Saul probably expected David to carry out a series of attacks , perhaps against men who were working in the fields. It would be easy to kill the first few men, but after that, his enemies would be watching for him. Saul hoped that David would become more desperate. Then David might take greater risks.
Saul was a very cruel man, but  David was not cruel . David considered it murder to kill a man who could not defend himself (2 Samuel 4:11). Murder is against God’s law (Deuteronomy 5:17), and David wanted to obey God completely (Psalm 40:8).
We do not know how David killed these Philistines; but he acted in a wise manner. A likely explanation is that a group of several hundred Philistines entered Israel to rob the country. David and his men were ready for them. David led his men in the attack and they killed 200 men. After the battle, David’s men collected the foreskins from the dead bodies. The number of foreskins was therefore  double what Saul had requested.
So David paid the price, and Michal became his first wife.