Gender identity is a person’s internal knowledge of their gender – the deep knowledge that they are a man, a woman, or non-binary.
Everyone has a gender identity, including people who are not trans. Men who are not trans, for example, still know internally that they are men. That’s their gender identity. For a trans person before transition, there may be a mismatch between the gender they know they are and the one people around them assume they are.
For more information on transgender people and the trans movement, check out the National Center for Transgender Equality.