"To deepen the flavor" means to enhance or intensify the taste of something by adding richness, complexity, or intensity to its existing taste. Full definition
Maple syrup is a little bit of a luxury item, but it does deepen the flavor of these muffins. (tofufortwo.net)
I would have never thought to use soy sauce in a tomato sauce recipe, but it really does deepen the flavors. (emilybites.com)
Adding garlic or spices or herbs can deepen the flavor profile, add color and interest, and help you find culinary expression in your own kitchen. (ournourishingroots.com)