DisplayFunction is an option for graphics and sound functions that specifies the function to apply to graphics and sound objects in order to display them.
The default setting for DisplayFunction in graphics functions is $DisplayFunction, and in sound functions is $SoundDisplayFunction.
A typical setting is DisplayFunction->Display[channel, #]&.
Setting DisplayFunction->Identity will cause the objects to be returned, but no display to be generated.
See The Mathematica Book: Section 1.9.3 and Section 2.9.14.
See also: Show.