Cream of tarter is sodium bitartrate and it's often used as a stabilizer in meringues and angel food cakes. For snickerdoodles, however, it adds a tanginess. It also helps with the chemical reaction of your baking soda to make the chewier texture for these tasty cookies. If you don't have any on the shelf you can substitute 2 teaspoons of lemon juice for every teaspoon of cream of tartar that a recipe calls for.