All active elements in analog signal-processing sort of 'bend' the curve shape and by doing that cause the appearance of harmonics*.
These disturbances in the signal can be detected with a predifined input signal (usually a sinusoid wave), analysis of the output signal the added harmonics in the waveform can be detected. Total harmonic distortion or THD is the percentage of the added harmonics compared to the total output signal.
This phenomenon is audible, depending on the kind of music, from about 0.3%...3%. Modern audio equipment usually has a value of less than 0.1%
* Allthough this is a measure used in audio processing, the same effect appears in any kind of analog signal processing.