Phasing occurs when a sound source arrives at two different mics at two different times, due to the differences in distance from the mics to the sound source. If the time difference is 1/2 the wavelength of a particular frequency of the sound, those frequencies, when the sound of the two mics is combined, get cancelled. The result of some frequencies getting cancelled (and others reinforced) is called comb filtering.
This is not the same as polarity reversal, which is usually caused by a wiring error - resulting in 180 degree reversed waveform (mirror image) which, in a stereo situation, can also cause drastic (or even complete) cancellation, especially if the stereo is summed to mono.
The rule of thumb for minimizing phasing effects is to have your mics at least 3x the distance from eachother as they are from the sound source. This is commonly called the 3:1 Rule. (Example: you have two mics on a piano that are 8" away from the strings. Using the 3:1 rule you want the two mics to be at least 24" from eachother.) The obvious exceptions are if you are doing certain specific stereo mic'ing techniques, like coincident pairs, MS, etc.
Next lesson: "How to use the Cap Locks Key"
