A quadrature relationship refers to the relationship between two signals that are 90 degrees out of phase with each other. In audio waves, this typically refers to the relationship between two sinusoidal waves that have the same frequency but are offset by a phase angle of 90 degrees.
The quadrature relationship is important in many aspects of audio and signal processing, such as in the creation of stereo sound, where two signals that are in quadrature with each other are used to create a sense of spatial separation between sounds. In addition, quadrature signals can be used in various modulation techniques, such as amplitude modulation (AM) and frequency modulation (FM).
