Being "sexist" means that you think that men and women can't or shouldn't be allowed to do the same things.
People usually call each other "sexist" for thinking that women shouldn't be allowed to do the things that men do. It's uncommon for someone to be accused of being "sexist" against men. You may hear a man call a woman "sexist" as a joke sometimes, though.
A: Honey, it's your job as a man to make money and support me.
B: What? That's sexist!