This phrase, like the similar "ועבדהו בלבב שלם," emphasizes the importance of serving Hashem with sincerity, full devotion, and wholeheartedness. It suggests that one's service to Hashem should come from a place of genuine commitment, where the heart and mind are fully engaged in the worship and commandments. The idea is to cultivate a relationship with Hashem that is not just about external actions but involves deep, heartfelt dedication.