Definition of icing noun from the Oxford Advanced Learner's Dictionary

icing

noun
ˈaɪsɪŋ
 
; ˈaɪsɪŋ
 
(especially British English) (North American English usually frosting) [uncountable]
 
a sweet mixture of sugar and water, milk, butter or egg white that is used to cover and decorate cakes
see also royal icing
Idioms

the icing on the cake

; (US also)

the frosting on the cake

something extra and not essential that is added to an already good situation or experience and that makes it even better
It's an added bonus—the icing on the cake.