What do in phase and out of phase mean?
Phase refers to the relationship of two sine waves (signals) to each other. If both signals are at their highest peak (+) at the same time they are in phase. If one signal is at its highest peak (+) while the other signal is at its lowest peak (-) they are 180 degrees out of phase. While a complicated and lengthy explanation of this phenomenon is possible, guitar electronics are fairly simple and a basic understanding goes a long way to ensuring your guitar works properly.
Humbucking pickups have the individual coils connected in opposing phase and each coil uses opposite polarities of a single bar magnet. If either of these two conditions is not met the resulting tone will be thin and nasal sounding. It is the combination of these two elements that give a humbucking pick up it’s strong full tone with the majority of hum being cancelled The same thing applies to single coil pick ups. It is the combination of opposing phase and magnetic polarity between a RwRp single coil and a standard single that causes the combination to be noise canceling. You can find more information here on phase and polarity.