What is GIF Dithering Option?

You can set up the GIF dithering option on Setting. When you save the file as GIF with dithering option, it can make your GIF more natural.

GIF file format has a 256-colors limit, so when you make a GIF file from a true-color image, it can be perceived that the quality gets lower because of color reduction. In order to minimize this problem, we provide the dithering option. Since the result can be varied depending on the type or situation of the original file, you can use the dithering option however you like. 

Reference
Dither - Wikipedia