What does 1 Samuel 17:56 mean?

Saul has asked the commander of the army about David's family (1 Samuel 17:55). This conversation apparently takes place as David is approaching the field of battle to fight Goliath (1 Samuel 17:40). King Saul is impressed with young David's bravery and confidence in the Lord to give him victory. Nobody else had volunteered to fight the Philistine champion, including the king. Saul would also need to know who David's father was—to notify him of David's death or to reward his household by relieving them of paying taxes should David prove victorious (1 Samuel 17:25).

Saul knew David, who both played the lyre for him and served as armor-bearer (1 Samuel 16:14–23; 17:15). Saul had also spoken with David this day (1 Samuel 17:31–40). But he apparently did not recall that David was the son of Jesse (1 Samuel 16:18; 17:12–15). Saul has asked the commander of his army, Abner, who David's father is (1 Samuel 17:55). Since Abner did not know, Saul now asks his commander to find out. The king will draft David into his permanent service, no longer allowing David to return home to Bethlehem to watch his father's sheep (1 Samuel 18:2).