Everyone has a favorite function. We develop one of the four mental functions to a greater degree than any of the other three. This first and favorite function is like the captain of a ship, having the most important role in guiding us, and it becomes the type core of our conscious personality. This is called the dominant function. During the first part of your life, you come to rely on your favorite function, and you tend to develop the most skill with it.
Some people, for example, give the most weight to their Intuition. They trust that function the most, and they are the most energized when they use it. As children, they probably tended to focus on Intuition (assuming their family supported it), and they probably became involved in activities where they could use their imagination and focus on possibilities.
Functions develop by being used consciously and purposefully for things that matter. As the dominant is used, it becomes strengthened and differentiated from the other functions. We tend to have the most skills and conscious use of this function, and we tend to trust it the most.
Extraverts use their dominant function in the outer world because, by definition, they prefer to live in the outer world. They put their best foot forward.
Introverts use their dominant function in their inner world because, by definition, they prefer to live in their inner world. Their development of the dominant function is less visible.