Jack and Phoebe have an interchange about saying grace over lunch. What is that interchange, and why is important to the story?

Phoebe asks Jack if he wants to say grace over the food, and when Jack says no, Phoebe becomes alarmed. Her mouth opens, her eyes grow wide. When Jack sees this, he immediately tells her he has already said grace in his head. With this, she calms down and smiles. In this scene, we see how much Jack cares for Phoebe and does not want to hurt her.