>>3732868phase of a waveform is where it starts on the cycle, it's measured in degrees or radians, pic related is where the waveform would start it's oscillation with 180 degrees (pi radians) phase.
When it comes to sound design phase is important because of 2 things.
1. Phase Cancellation: a phenomena that occurs when 2 waveforms are combined with differing phases, the most extreme of which results in complete cancellation (total silence), this video explains it quite well
https://www.youtube.com/watch?v=UoRo80VzSEY2. Doppler Effect: This is the basics of digital FM synthesis, and it happens when you change the phase of a waveform while it's playing. The reason the doppler effect works is because the source of the sound is moving towards or away from you, this changes it's phase and causes the pitch to change. This pitch change would follow the change in curvature of the movement.
There are more applications in audio engineering but those are the most important in my opinion.
Oh, and stereo phase changes form a mono can give the allusion of a stereoised sound.
