C’est la cerise sur le gâteau, literally “It’s the cherry on the cake”, refers to something that makes it better or more appealing, but not obligatory. It’s the same for the cake, it would be good without the cherry but with a cherry on top, it would be more appealing! And in English? Well, it’s “the icing on the cake”!
But be careful! It can also be used in an ironic sense too!