Meringue powder is a shelf-stable dried mix used in place of fresh egg whites in meringue-based recipes, made primarily of dried egg whites, sugar, and stabilizers such as cream of tartar or gum arabic.
It is reconstituted with water and whipped the same way fresh egg whites would be, producing a stable, glossy meringue suitable for piping, pavlova, meringue cookies, and royal icing. It is popular in commercial baking and any setting where using a large volume of fresh egg whites, or food-safety concerns around raw egg whites, is impractical.
Meringue made from the powder is generally sturdier than fresh-egg-white meringue but can differ slightly in texture and volume; some brands add vanilla or other flavoring.