Cream Of Tartar is a baking ingredient that is used sometimes used as the acidic ingredient in some brands of baking powder, as well as being a vital ingredient in making playdoh at home.
However Cream of Tartar is most used, as we use it at home, for stabilising egg whites so Axel would add a teaspoon to egg whites as he is whisking them to make meringues or you can add it to icings to give it stability.
Cream of Tartar mostly comes from the sediment produced in making wine.