What does 1 Samuel 18:27 mean?

Because David would not accept the hand of Saul's daughter as a gift (1 Samuel 18:18), the king has come up with a challenge. David can prove himself and earn a suitable bride-price by killing a hundred Philistine men. Proof of each death would be the enemy's foreskin: the flap of flesh which is removed from the penis during circumcision. Since Jews were circumcised as infants (Genesis 17:9–12), the only source for these are Philistines, who won't offer them willingly (1 Samuel 18:25–26).

It's important to note that Saul's request isn't to indiscriminately murder Philistine citizens. The Israelites are at war with a people constantly attempting to expand their territory and enslave Israel's people (Judges 10:7; 13:1; 1 Samuel 4:10; 13:5). David has been put in command of a battle group (1 Samuel 18:12–13). The challenge is for David to fight until he has killed one hundred men of Israel's great enemy. Even in ancient wars, most soldiers were not killed in a typical conflict. To complete the challenge, David must pursue something more than "just" victory on the battlefield.

Saul apparently makes the challenge even more difficult by including a time limit: David set out to accomplish the task before a deadline expired (1 Samuel 18:26). This would avoid David being able to kill a few Philistines at a time to eventually reach a hundred total.

But David isn't wasting time looking for loopholes. He chooses men, goes to where the Philistines are, and kills two hundred, rather than the one hundred required by Saul. This fits the pattern which characterized David's life and personality: boldness, courage, and defying limits on what is possible when the Lord provides and protects. In obedience and faithfulness to the Lord, David kills many men in close and bloody combat during his life. This warlike lifestyle serves God in those specific contexts (Psalm 144:1). But it also makes David an awkward choice to build a temple for God. Instead, that will be assigned to David's son (1 Chronicles 22:8–10).

Collecting the foreskins of the dead must have been a gruesome task. Other cultures have done the same with scalps of hair, ears, noses, and so forth. Taking these trophies would have been in keeping with the up-close nature of war during this time. Since the foreskin must be cut from the penis, one can argue it's an even more direct proof of death than an ear, nose, or hand. There is also irony in using a central symbol of Judaism to tally victories against a pagan enemy (1 Samuel 17:36).

David returns to Saul and presents him with the enemy foreskins. Saul may have been disappointed that David wasn't killed, but He honors the agreement and gives his daughter Michal (1 Samuel 18:20) as David's wife.