Cream of tartar is is an acid used to stabilize egg whites when you whip up meringue to make a meringue frosting or a meringue-based cake. 

When you use cream of tartar in your whipping meringue, the acid in the cream of tartar keeps the egg white foam, moist and stretchy. so it can whip as far as it can ( maximum volume) and retain air longer than whites whipped without cream of tartar. 

It also adds some strength if you are using older eggs, I use cream of tartar as often as I use baking powder or baking soda.

If you don’t have it at home, you can substitute it with, acid-like fresh lemon juice, but I do highly recommend buying a bottle of cream of tartar and have it in your pantry and use it whenever you whip up meringue, You be surprised what a difference it makes!

Cream of tartar is sold at most supermarkets in the spices aisle or you can buy it online at amazon.