Phase basically refers to waveforms and the difference in time vs. amplitude (loudness) between two sources.
Have you ever recorded a drum set or other instrument with multiple microphones or duplicated an audio waveform in ProTools and tried putting it in sync with each other but was slightly off? when you played it back it might have sounded very weird, what many people describe as a "hollow sound". This is what a phase issue is!
Below, in diagram "a" you can see 2 waveforms that are perfectly in sync with one another (the 2 waveforms are 360 degrees in phase), this will result in an even louder audio waveform.
In diagram "b" you can see 2 waveforms that are not in sync with one another. You can also see that the peeks of each waveform are pointing towards each other, this means they are 180 degrees out of phase and the 2 waveforms will completely cancel each other out, meaning no sound will be heard!
So why do we encounter phase issues? In the recording studio when you place multiple microphones to record an instrument, depending on the placement of the microphones, when we record, the actual sound from the source may reach each microphone at different speeds since there is a speed to sound (roughly 1,100 feet/sec). The more microphones that are used in a recording, the higher the probability for phase issues. Phase issues can be fixed by simply listening to all the microphones together in mono before actually recording. one can detect phasing issues easier when listening to a mix in mono, If you do encounter phasing issues try fixing your mic placement or flipping the phase on the channel input of the mic. If its to late to fix phasing issues during the recording process, during mixing, one can also nudge the waveform by a couple milliseconds, this can make a big difference!