What does 1 Samuel 18:3 mean?

Jonathan, Saul's son, is deeply impressed with David. The two of them have much in common. Both are bold soldiers with a devoted faith in God. Early in Saul's reign there had been a standoff with the Philistines. Things looked grim for Israel. Then Jonathan attacked the enemy with only his armor-bearer and complete confidence in the power and faithfulness of the Lord. The Lord used Jonathan's courageous action to spark a panic among the Philistines that led to Israel's overwhelming victory (1 Samuel 14:1–23).

Jonathan had doubtless been present for Goliath's forty–day-long challenge to the Israelites to send a man out to fight him to decide the battle. It's unknown why Jonathan didn't accept the challenge himself. Perhaps he, too, was terrified of the giant. Maybe Saul refused to risk his heir. Undoubtedly, Jonathan resonated with David's declaration that the Lord meant to show all the world there is a God in Israel (1 Samuel 17:46–47).

Jonathan's response to David's bravery and confidence in the Lord seems instantaneous. Here he formalizes that admiration and love for David into a covenant. The vow isn't romantic. The sense of the covenant seems to be that Jonathan pledges himself to fight with and serve alongside David. Soon, Jonathan will fulfill his pledge when he understands the depth of his father's hatred toward David. Jonathan will defend David's honor, beg for David's protection of his own family, and send David away from his father's murderous rage (1 Samuel 19).