It's exactly what it says on the tin: That traditional male and female roles in society became reversed, with mares as the breadwinners and stallions as the home makers.

How did this happen?
All up to the story writer. The most common route is to say that the mare-favoring gender skew turned stallions into the softer and more protected gender.

Where did this come from?
It's hard to pin down. The concept sprung up and remained a niche for a long time. Recently, it's gained a bit more traction.

What will be posted here/What can I post here?
A common theme is to have a human character featured somewhere to shake up the established roles in Equestria, however, this is NOT a requirement. So long as a story features RGRE elements, it's at home here. It does not have to be limited to Equestria, either.

Why?
It's both an amusing concept and allows dialogue about subjects that are touchy in the modern age, but with pony flavor.