I Samuel 23:7 Now it was told Saul that David had come to Keilah. And Saul said, “God has given him into my hand, for he has shut himself in by entering a town that has gates and bars.” 8 And Saul summoned all the people to war, to go down to Keilah, to besiege David and his men. 9 David knew that Saul was plotting harm against him. And he said to Abiathar the priest, “Bring the ephod here.” 10 Then David said, “O Lord, the God of Israel, your servant has surely heard that Saul seeks to come to Keilah, to destroy the city on my account. 11 Will the men of Keilah surrender me into his hand? Will Saul come down, as your servant has heard? O Lord, the God of Israel, please tell your servant.” And the Lord said, “He will come down.”

In Old Testament Isreal, God made ways for communication to take place between Him and His people. One such way was through the priest using this special ephod.  This was especially used when the answer to a question would affect the whole nation. David knew this and sought an answer to a pressing question: "Was Saul's intention to come fight his fellow countrymen and destroy David and his faithful fighting men?" God answered clearly.  David knew what he needed to do: promptly withdraw.  Sometimes we just need to ask God out questions clearly and concisely.  He will answer clearly as well, showing He has heard.