Imagine a cake. The "icing" or "frosting" (in the US) is the sugary glaze that goes on the top to add a sweet flavour. So, in metaphoric terms, the 'icing on the cake' is something further good on top of already good news. "On Monday, I found a £10 note in the street, I was able to park at work, and the icing on the cake was when my boss told me I could go home early".