Detune is a parameter that is used to offset a played note from its base frequency. The unit for detune is cents; one cent is equivalent to 1/100 of a semitone and there are 12 semitones in an octave.
By detuning in cents rather than hertz, the percieved difference in pitch is normalized to whatever note is being detuned. A 10hz detune sounds much more dramatic on a low note with a lower frequency than a high note since it is a higher percentage of its frequency.
The equation for producing an output frequency from a base frequency and a detune matches that of WebAudio, taken from the WebAudio spec: https://www.w3.org/TR/webaudio/#computedfrequency
computedFrequency(t) = frequency(t) * pow(2, detune(t) / 1200)