What does that mean as a porn category?
If you don't know I ain't telling. About 20 years ago I saw a license plate with it, and asked my teenage kids what it meant. That was the answer they gave me. Isn't the internet grand.
Everyone already covered it, but my answer now is younger version of G-MILF.
Mothers I'd Like To Fuck. In other words, women of one or more prior generations to the observer, yet still hot.
Sorry, but you asked.
It is not at all limited to the porn world, in fact, I suspect that the porn world appropriated it from some broader context.