Gender is socially constructed and because of this gender roles and perception of gender are different in every society and can even change in a society over time. Like how we see certain things, like colors (in America), as gender specific when in reality they're gender neutral. Pink is feminine and blue is masculine in modern America, though that was not always the case and even now that view is changing again.
Gender is based off of our perception of biological sex in regards to femininity/masculinity and what it means to be fem/masc (the roles, behavior, etc.) in the society in which one lives in. Gender expression is the outward manifestation of one's self and can align with the societal norm and expectations of sex and gender (cisgender individuals) or it doesn't align (transgender, agender, gender fluid individuals, etc.).
I left out some details but that's the gist of it. Whatever gender someone identifies as and how they express it should be respected.
