This is a cute little idiom that has a similar meaning to the “cherry on the cake”.
What it means:
The icing on the cake refers to that little extra that makes the whole thing even better than it already was.
When to use it:
I sometimes use it to encourage my students during the speaking for example. Imagine Fulanito did a little presentation and there were very few mistakes. I could say “well done, almost no mistakes, and the icing on the cake…you used connectors!”.
Other interesting idioms in I:
In a flash – very quickly, immediately
In cahoots – This means that you are in an alliance or partnership…but that you probably are up to no good!
In the red – when your bank account is empty or under 0…like at the end of every month for example.