RWRP, or "reversed-wiring reversed-phase", is a term usually used to describe single-coil pickup sets or individual single-coil guitar pickups.
First, what do we mean when we describe a guitar pickup's phase? Essentially what we are discussing here is the relationship between the guitar string's movement and the electronic output from the pickup. If you imagine a sin wave to simplify, sometimes that wave is positive, and sometimes it is negative. You could imagine "flipping it over" so that it is now negative when it used to be positive. If you took one wave that was positive, and added it to the negative "flipped-over" version of itself, they would basically cancel out.
The same thing happens with guitar pickups - ever wire two pickups out of phase by accident, or on purpose if you are Peter Green? What you'll find is that you get less output overall, and most of the bass has been sucked out. Without getting too technical, that is because the pickups are in two different places on the guitar body, so they "see" different parts of the vibrating string, and the lower in frequency the sound the more likely that the pickups are seeing the "same thing", so when their phases are reversed, they will cancel. I recognize diagrams would be helpful here - I'll add them later.
So anyhow, generally you want your pickups in phase with each other. Two things define their phase:
1. The polarity of their magnets (for a Stratocaster this would be whether the magnet pole pieces are "North" on top or "South" on top.
2. The direction of their winding (i.e. if you looked at Strat or Tele pickup from the top, if the pickup was wound in a clockwise or counterclockwise direction).
So to be in phase two pickups need either the name magnet direction and the same winding direction, or a different magnet direction AND a different winding direction. Having a different magnet direction AND a different winding direction is called RWRP or Reversed Wiring (the clockwise or counterclockwise wiring) Reversed Phase (the magnet).
Ok fine, so we have two choices here, so why ever bother with RWRP? Well, as anyone knows who has ever tried to record with, play high gain through, or even look funny at a single-coil pickup, these pickups have a lot of noise. That noise also has a phase, but the only thing that changes the noise's phase is the direction of the winding. So if you have two pickups wound in different directions, and you combine the two, the noise from both will be out of phase, more or less cancelling out. But as we mentioned before, you cannot just reverse the wiring, you need to reverse the magnet as well for playing - the noise doesn't "notice" this though, and so you get your noise cancelled!
How do guitar makers put this to work? Simple, they RWRP the middle pickup of a Strat, or usually the neck pickup of a Tele. For the Strat, that means positions 2 and 4 give you humbucker-like noise cancelling, and for a Tele your middle position cancels noise.
Some will claim that this isn't vintage-correct, and that you lose some "highs" doing this...I simply don't believe it. The only explanation that I find credible that would explain a difference in tone (unless you like the sound of your noise) is if your two pickups were close enough that their magnetic fields interacted, then, and only then, can I imagine there would be a difference in tone. Otherwise, RWRP is a free lunch.
Note: When mixing pickups from different manufacturers, one's RWRP may not match another's. You need to find out from them (for a single coil) whether the top part of the magnet is north or south, and the direction the coil is wound (specify an orientation, as in when viewed from the top). If you are concerned about this, you can just buy from a single manufacturer - odds are you want a homogenous sound between your 2-3 pickups anyhow.