This question is usually asked in bad faith, but I'll bite.

Gender ideologues or genderists believe that whether someone is a man or a woman is determined primarily by that person's gender self-identification ("A woman is anyone who identifies as a woman").

This in contrast with the classic belief, held by radical feminists, J. K. Rowling, and many others, that whether someone is a man or a woman is determined primarily by the physical attributes of their bodies related to biological sex (genetics, hormones, gonads, etc.)

As well as making the claim that woman and men are merely identities, most genderists also rely on sexist stereotyping to support this belief.

Such as, displaying a photograph of a man wearing makeup, styling his hair long and wearing attire more typically worn by female people, and asking incredulously, "so are you saying she isn't a woman?!"

Or citing childhood memories recalled by men who call themselves women, of "playing with Barbies" and suchlike, supposedly on the basis that boys don't do that so they must be girls.

Genderism isn't just a ludicrous, reality-denying ideology, it's fundamentally sexist too.