Aging egg whites is the process of allowing your egg whites to sit partially uncovered for 24 hours or longer to allow some of the liquid to evaporate out, therefore drying the egg whites. This is meant to help you achieve a sturdier meringue. Most of the time I do not age mine, but I do recommend it in my macaron recipe just to be safe. If you're a beginner, if you've struggled with your meringue in the past, or if you're in a more humid environment, I would recommend aging.