Every personality has a dominant function. If you're Extroverted than you show it, if you're Introverted you don't, showing your secondary instead.
It gets a little complicated but the full chart is here: http://www.oursmallhours.com/2011/10/myers-briggs-type-indicator-type-dynamics-and-hierarchy-of-functions.html
So, for example, I've always thought I was an INTJ (may not be), but for sake of example,
- Introverted Intuition is the Dominant Function
- the secondary (Auxiliary) is always the other middle letter, so in this case Thinking, which would be Extroverted (since INTJ's are introverted
- to find your Tertiary, it's the opposite of your secondary
- and your Inferior is the opposite of your dominant
Dominant: "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."
"The auxiliary can be thought of as the first mate on the ship where the captain is the dominant function. The auxiliary function tends to develop after the dominant function. During adolescence and early adulthood, individuals come to develop skills in and rely on their dominant and auxiliary functions. They give less attention to the opposite functions, the letters that do not appear in their type."
"Development of this function tends to come later in life (about midlife) after you have grown and feel comfortable with the dominant and auxiliary. As you grow and develop, you learn that there is a time and place to use your third and fourth functions."