Exactly what makes a MOPAR a "Matching Numbers" car?

I think a lot of guys that grew up with these cars would love to have one with all it's original parts still in tact just like they drove them when they were young.
To offer a different perspective, I'm 67 and I did grow up with these cars and we spent most of our time and money taking the factory parts off and replacing them with stuff from the speed shop :rofl: