Imagine you are a King's messenger soldier. The King sends you to go check if the enemy is coming. You go there, you see the enemy, but you come back and say: "No, King, everything's calm, you can go to sleep!" What's going to happen? The enemy is going to attack, and the King is going to lose, because the messenger lied.
The Bible says God wants us to learn to be truthful. Studying the Bible and listening to Dad is training to become trustworthy messengers. When someone asks you something, you should tell the truth, not make up a story. That way, everyone can trust you!