One of the secrets of great image manipulation is to separate images into components. Frequency separation is often mentioned, breaking the image into so-called high-pass and low-pass layers. The high-pass layer separates out the sudden changes in an image and fiddling with this layer can enhance sharpening or even out blemishes. The low-pass layer can be used for smoothing out color transitions. Portrait photographers use this extensively for smoothing skin and removing spots. There is no native Frequency separation tool or filter built into gimp. Doing it requires several time-consuming steps and it is easy to get the steps in the wrong order, turning the image into a hopeless mess.
Another separation technique separates color from luminosity. The human eye is far more sensitive to relative luminosity than color, which is why Andy Warhol images still make sense to us. Playing with the color without affecting the luminosity can produce beautiful artistic effects and are the technique is particularly well-suited to landscape photography. And again, there is no native way to perform the separation.
This script puts two new filters in Filters/Generic/Split... Which will do the separation for you. It requires layer-groups so will only work in gimp 2.8 and newer.