Waveforms are repetitious. That is, they proceed through regular cycles. Phase is defined as to how far along a cycle the waveform is, with 360 degrees being the point of a completed cycle. Most engineers take phase into account while capturing an audio source with multiple microphones, since audio hitting different microphones even with a small delay, can put the summed audio out of phase, which results in certain frequencies of the original sound source being attenuated or even cancelled. In fact, if a waveform is combined with an exact copy of itself but with the two being 180 degrees out of phase, they will cancel each other out completely.