RWRP stands for Reverse Wound, Reverse Polarity. When the middle pickup of a single coil Strat set is RWRP, it cancels hum in switch positions 2 and 4, retaining the iconic “quack” tones while cancelling 60 cycle hum and noise.
For more in depth discussion of pickup wind and polarity, check out this article: Pickup Polarity and Phase Made Simple