The icing on the cake is something that makes a good situation even better.
e.g. I got a distinction for my dissertation, but the icing on the cake was that my supervisor wants me to present with her in a conference.
However, the phrase can be used ironically (to mean the opposite); the icing on the cake can be something that makes a bad situation even worse.
e.g. So, this morning my alarm didn’t go off, there was no hot water, I missed the bus, got caught in the rain, arrived 20 minutes late, but forgetting my notes for the seminar was the icing on the cake.