Icing on the cake is an extra benefit or delight that makes a great situation even better. Acing that difficult exam was awesome; getting dinner from your favorite restaurant afterward was the icing on the cake.
Another form of this term is the frosting on the cake. If you were given a cake without icing you'd probably be pretty pleased just to have a sweet snack, but you might be even happier if it did have icing! The phrase can also be used in a sarcastic way: "I'm late for work because my car got a flat tire, and the icing on the cake is I tore my shirt trying to change it."