In Glossary Terms by phoenixgreen

Audio phase occurs when two or more microphones are recording the same audio signal.

In Phase means that the replicated audio tracks are exactly the same resulting in louder audio.
Phase Cancellation means that the replicated audio signal is exactly the opposite position in the cycle to the original. The outcome is that the signal is cancelled out completely
Out of Phase means the replicated audio is slightly in front of or behind the original signal resulting in a complicated wave form