Phase is defined as how far along its cycle a given waveform is. As we mentioned before, sound waves are periodic, or cyclical. From your trigonometry class back in high school, you may remember some evil sine-wave graphs where one period equalled 360 degrees. It is possible to have two identical sound waves of the same frequency and amplitude, but one is delayed slightly-we term this being "out of phase" with respect to each other. If you have one sound wave, it doesn't much matter how far along the sound wave is at a given instant. However, when you have multiple sound waves which are "out of phase," or delayed slightly with respect to one another, the waveforms will interact with each other in constructive and destructive ways. How much the waves interact, and at what frequencies they interact, depends on the waveforms involved, and how far out of phase they are-- two identical sine waves, 180 degrees out of phase with respect to each other, will cancel completely. Draw it on a piece of paper-- add one sine wave with a positive peak at 90 degrees and a negative peak at 270 degrees to another sine wave with a negative peak at 90 degrees and a positive peak at 270 degrees-- whaddya get? Nuttin'.
Conversely, if two similar waveforms, of same frequency, shape, and peak amplitude, are added, the resultant frequency is of the same frequency, phase, and shape, but has twice the original peak amplitude ((y = sin x ) + (y = sin x) = y = 2 sin x). We call this state where two waves are exactly the same in-phase.