If you often read articles with mathematical formulas, you might want to play with settings here, but the standard setting (“HTML if very simple or else PNG”) usually works fine. It shows complex formulas as an image, which may make them more readable than the usual text display. But check out the alternative, “Recommended for modern browsers”, which you may like better. For text-based browsers like Lynx, there is the “Leave it as TeX (for text browsers)” option. (“TeX” is a typesetting system that works very well for complex mathematical formulas.)