When you already have it good and get something on top of what you already have.
This idiom is generally applied in scenarios where one already has something good and great to his part and something more gets added to it.
'Icing on the cake' means adding much more favorable conditions to the already existing scenario.
1)Getting 94% was really a great achievement and standing university second was like an icing on the cake.
2)Increment in the salary was very much exciting and getting promotion at the same time is like an icing on the cake.
3)Learning java language is itself appreciable and applying it in the real-time and receiving accolades came like icing on the cake.
4)Decoding the formula and inventing an element was a great achievement and thereafter, receiving Nobel Price really came like icing on the cake.