It's kinda hard to explain without pictures... You know a sine wave is the simplest wave. So imagine a sine wave coming from a speaker. It goes up and it goes down over time. Let's say a 1 hz sine way goes above the center and then below the centerline and then returns to the center line for one complete cycle in one second. So over the course of time it's going up and down, up and down. Now suppose one micrphone is a foot away and another is 18" away. It could happen that at the same moment the sine waveis hitting the first mic on the up stroke of the cycle while the sine wave is hitting the down sroke of the cycle at the second microphone. The two signals would be reversed and would cancel each other.
Some cars put a microphone at the tailpipe to monitor the sound. then they replicate the sound but with an opposite phase. The whole car beomes quieter.
Here's an article that at least has some pictures and definitions:
Phase: Definition from Answers.com
The easiset way to handle it in recording is to reverse one of the phases and see if the signal gets stronger or not.