The Animated GIF image format is made to display a maximum of 256 colors in one image. When an image containing more than 256 colors is saved as a GIF, the color limitation of the GIF format creates artifacts (defects) such as Dithering (e.g. post #720057) or Color Banding (e.g. the background of post #76332).
Note that despite artifacting in high-color images, GIFs have legitimate uses, such as for Pixel Art, low-color Oekaki images, and animated images.