It is not cream of tarter, which, as you know, is often added to egg whites as you whip them to make them more stable. Unlike plain powdered egg whites, meringue powder is made of dried egg whites, sugar, and gum. We’re guessing that your recipe calls for meringue powder and omits the egg whites entirely - presumably it’s just the powder and water.

Recipes that call for the powdered egg whites or meringue powder do so because of the growing fear of consuming raw eggs, as both products are pasteurized.