Graxpert is designed to remove gradients from astroimages. As the name suggests, gradients are gradual changes in brightness that are not part of the astrophoto but are caused by external interference. Causes can be eg light pollution or incorrect or missing flat correction, but also natural brightness gradients of the night sky and peculiarities of the optics used (shading in the form of a vignette).
Siril has built in gradient removal tool which I’ve heard is based on Graxpert code. However, the latest beta version of Graxpert has an AI mode which makes it even easier to use. You can download this from their Github page
Graxpert now has a new method for background extraction based on AI, which does not require the selection of background sample points. To use it, you simply have to choose the AI version in the advanced menu on the right and select ‘AI’ as the interpolation method in the menu on the left. The download and installation of the AI model is then automatically handled by GraXpert.