Anisotropic filtering (AF) is a feature of some video cards that sharpens the details of the fading-away part of a 3D object that recedes into the distance. Think of the text in the titles at the beginning of the Star Wars movies that is presented in large letters and then scrolls back into the distance. As it scrolls off, it becomes fuzzy and hard to read. In a 3D image, you may want a comparable effect to retain the sharpness of an object as it recedes; anisotropic filtering does this. (Isotropic describes objects or image elements with vectors of equal value along different axes, such as squares and cubes. Anisotropic describes objects whose vectors are unequal, such as trapezoids and parallelograms.)
Since anisotropic filtering requires intense processing as image frames are presented to the display, it may affect performance. A user may want to weigh the perceived improvement in visual quality against the effect on performance.