"The sculpture shows a scene from the Old Testament First Book of Samuel. The Israelites are at war with the Philistines whose champion, Goliath, has challenged the Israelite army to settle the conflict by single combat. The young shepherd David has just taken up the challenge, and is about to slay Goliath with a stone from his sling:
"When the Philistine [Goliath] arose and came and drew near to meet David, David ran quickly toward the battle line to meet the Philistine. And David put his hand in his bag and took out a stone and slung it and struck the Philistine on his forehead. The stone sank into his forehead, and he fell on his face to the ground.
"Typical shepherds attire only is David's clothing. At his feet lies the armor of Israel's King, Saul, given to David for battle. The armor was shed, as David was unaccustomed to it and he can fight better without. Also at his feet is his harp, often included as an iconographic device of David in reference to David the Psalmist and being a talented harpist." -- Wikipedia
Donatello + Michelangelo + Bernini