The best known use for cream of tartar is to stabilise egg whites when making meringues or meringue toppings. The cream of tartar not only stabilises the egg whites and allows them to maintain their texture when whipped into stiff peaks, but it also increases their tolerance to heat, which is very helpful, say, when you put a meringue topped pie or a baked Alaska into a hot oven. This allows them to brown nicely, hold their shape, and to not melt away and expose the interior of these desserts. There are other food uses for cream of tartar, including stabilising whipped cream, preventing discolouration of vegetables which have been boiled, and preventing sugar syrups from crystallising. It can also be used as an ingredient which will help activate baking soda (hence you may see it in some cake recipes), and it also sometimes is listed as an ingredient in salt substitutes.