2 by default
it blurs the textures (in a VERY basic sense) thats what you'll see, is the further the texture is away, the more it will blur
i however set mine at 1.5 not only for higher FPS, but its much more realistic imo (but.. its realistic for me because i dont use AA or AF)
the lower the more FPS, but you'll notice all the textures become much more blurry
going to high however will make all the textures sharpened and not very pretty
basically its a texture filter