thats icing on the cake

Thats icing on the cake is a phrase used when something good is added to something else that is good, so that it is twice as good as it began. By saying icing on the cake, the cake is referring to one good (sweet) thing and when you add icing, it is just better but doesnt mean it wasnt pleasant and acceptable to begin with. When used, this typically refers to something already successful and something else just makes it that much better.