What does 1 Samuel 20:6 mean?

If your father asks where I am, tell him I asked your permission to go home to Bethlehem for a family sacrifice. If he says, Sure, that’s fine, then I’m safe. But if he gets angry, you’ll know he’s out to get me. 1 Samuel 20:6 - Modern Text Bible

(David gives Jonathan a cover story and a way to gauge Saul’s intentions based on his reaction.)