Phase is a measure of the relative longitudinal position of two superpositioned sound waves. Phase is measured in degrees, the total length of a cycle being equal to 360 degrees. When two sound waves interact, their relative positioning will have an effect on the amplitudes of various frequencies present in the sound. This wave interaction produces what is called an interference pattern.
In the simplest concrete example, we have two identical sine waves produced in an anechoic volume of air.
Where the crests (maximum positive amplitude) and troughs (maximum negative amplitude) of each wave coincide with those of the other, the two waves are in phase, and are said to reinforce one another. Their collective energy is additive, and the net amplitude is doubled. This phase reinforcement is called constructive interference.
Where the crests and troughs of the two waves are diametrically opposed to one another, the waves are exactly 180 degrees out of phase, and are said to cancel one another. Their collective energy is subtractive, and the net amplitude is zero. This phase cancellation is called destructive interference.