royal icing, it would probably be useful information to have.
Meringue powder is basically powdered egg whites. Not powdered egg yolks, which are also available in stores, which I know because I bought some thinking that I bought powdered egg whites...yeah, definitely not the same thing. Meringue powder is what makes the royal icing dry hard. You can find it at most craft stores like Michaels and Hobby Lobby or of course on Amazon.