Mipmapping is a way of compensating for small imperfections created by the computer’s graphics card that can appear when you use detailed textures on a small object. Mipmapping re-samples the original texture and creates a stack of smaller images.
This solves the problem of textures with small objects “flickering” created by the filtering process as the object gets further away. By using many “mipmaps” of the larger texture, the filtering happens on a much smaller level, which improves its overall appearance. See Use MipMaps.