Intersectionality is the idea that a black trans woman faces different oppression than the sum of a woman, a black person, and a trans person. It's generally understood that the best person to ask about this oppression is a black trans woman. It's not the idea that only a black trans woman can fight a black trans woman's oppression, though I too have seen people try to do that sort of gatekeepy nonsense. Asking people to explain the struggles they face and understand each separately is actually a powerful tool for building coalitions.