Okay kiddo, so a long time ago in theater, only men were allowed to act on stage. But sometimes there were roles for characters who were women. So what did they do? They had men dress up like women and play those parts!
But then, some plays started having roles for women who were pretending to be men within the play. These roles were called "breeches roles" because the women had to wear pants or breeches instead of dresses like they normally would on stage.
It was pretty cool because it showed that women were just as talented as men and could play any kind of character they wanted, even ones that were traditionally seen as male. Nowadays, in modern theater, it's totally normal for women to play any role they want, and there are even plays where all the characters are played by women! That's called gender-blind casting.