What does 2 Samuel 23:13 mean?

Once, during harvest season, three of the thirty chief warriors went down to David at the cave of Adullam. The Philistine army was camped in the Valley of Rephaim. 2 Samuel 23:13 - Modern Text Bible

(This sets up a story about a daring act by three of David's best men, while David was hiding from the Philistines.)