Anti-aliasing is the process by which your graphics card 'irons out' the jagged lines which appear more and more at lower resolutions. The thing to think about is that a computer monitor can never display a 'true' curved line because of the square pixels which make up the display. So, the higher the resoluton: the smaller the pixels and the 'truer' the curves which can be displayed. What anti-aliasing does is render the pixels along the edges of polygons, textures, and surfaces in such a way as to create the illusion of smoothness. It does this by altering the colour and tone of the necessary pixels. As this process works on a per-pixel basis it is very intensive and thus you need increasingly powerful graphics cards to do this.