Imagine that there are two big circles around everyone: one for boys and one for girls. These circles are called "spheres," and they're supposed to be separate from each other. They're like two different rooms that girls and boys are supposed to stay in.
The idea of separate spheres came about a long time ago when people thought that boys and girls were naturally good at different things. Boys were supposed to be strong and tough, while girls were supposed to be gentle and caring. Because of this, they thought that boys should do things like go to work and make money, while girls should stay home and take care of the house and the children.
So the two spheres were created: the "public" sphere for boys and the "private" sphere for girls. The public sphere was out in the world where all the important stuff happened, like working and making decisions. The private sphere was at home, where girls were supposed to take care of the house and children.
This way of thinking lasted for a long time, but it's not how we think anymore. We know that boys and girls can be good at any job they want to do, and that both boys and girls can be strong, tough, gentle, and caring. We no longer separate boys and girls into different spheres just because of their gender.