This filter is found in
The Noisify filter increases, for random selected pixels in active layer or selection, color of one or more channels. That's the equivalent of an added "noise" in sound field
This preview displays interactively changes before they are applied to the image. It reproduces the upper left corner of the layer or the selection.
These slidebars and adjacent input boxes allow to set noise level (0 - 1) in each channel. Apha channel is only present if your image holds such a channel. In case of a grayscaled image, Red channel is for Grey and Green channel is for Alpha.
When adding color to a pixel, its color may go beyond 255. In that case, a negative value is added and color value decreases. This may change color when value of the concerned pixel becomes lower than that of another channel.
If this checkbox is not checked, then all sliders move together and the same relative noise will be added to all channels so that the image color balance will be kept.