The gender binary is considered static. Male or female. Boy or Girl.
Society makes you feel like you need to be in one circle or the other.
For non-binary people, that gender binary is absent. In place of hard and fast boxes, there's a spectrum; of expression, of representation. You can place yourself wherever you want within that spectrum, or simply refuse to.
You can present as you want to. Feminine, masculine, androgeous. Whatever you feel represents you the most in that moment. And that idenity is fluid, and can change as you grow and change throughout your life.
You chose what you want to identify as, how you present yourself to other people, what pronouns you want.