"Retro" basically means backward. In the case of Retrofriction shifters, I would surmise that the name came from the concept that the internal spring provided a counteracting (backward) force to obviate slippage. In rocketry, it's a rocket that blows in the reverse direction.

I think for everything else it's just a marketing buzzword: I.e., anything that harkens back to an earlier stage or period.