Detune is one of the instrument options in Online Sequencer, which can change the speed at which a sound is played, changing the pitch. It functions exactly like speeding up and slowing down a record or tape, in which the speed can not be altered without also altering the pitch. Detune is usually used to extend the range of an instrument, change its timbre, shorten or lengthen the sustain time, or used it in combination with markers to create vibratos or slides.
Detune is measured in "cents". One semitone is 100 cents, one octave is 1200 cents. By default, the detune slider goes up or down by 100 cents at a time. To detune more precisely (microtones) hold Shift while clicking on the slider (you have to release your mouse click before you release shift). The detune slider goes from -1200 to 1200 cents. For more extreme (or preceise) detunes, you can use console commands.