Phase is the interaction of two tones generated simultaneously.
Phase and Familiarity: So far we've been talking about pure tones, which actually don't occur naturally in nature. So what do we hear in nature?
Complex tones occur in nature. They are combinations of pure tones, much like a brick wall is composed of bricks.
One difference is that the bricks in a wall are all the same size. The pure tones in a Complex tone are all different sizes, or more correctly, frequencies.
Not just any two pure tones will combine, moreover. They must be integral multiples of each other, like 125 Hz, 250 Hz, 500 Hz and 1000 Hz etc.
Actually if two pure tones of the same frequency and intensity are produced simultaneously, they will combine. If they are in phase (i.e.,the molecules move in the same direction at the same time) the result will be only one pure tone that is louder.