Gender expression is the ways you present gender, through your actions, clothing, demeanour, and more. Your outward-facing self, and how that’s interpreted by others based on gender norms.
If we try to describe masculinity or femininity, we find that it is highly dependent on your culture, your upbringing and who you’re asking. Blue for boys and pink for girls was not always the case!
Gender expression is all about how you conform (or don’t!) to these norms. We probably immediately jump to thoughts of drag queens however we all vary our expression on an almost daily basis. Just think about what you wear and how you act in private and how you act at a formal occasion, chances are one probably is more conventionally ‘feminine’ and one is more conventionally ‘masculine’ but that doesn’t define your sex or gender identity.
Not requiring people to conform to norms of expression frees everyone, from sexist uniform policies that discriminate against women to those who like to express their gender against the norm.