Gild the Lily: to spoil something that is already beautiful by adding something not needed
It's a nonsense phrase used when someone is upset or sad. You pat them on the back and say "There, there." You can also say "there, there, everything will turn out right."
It's a rather rude way of saying "be quiet" - it's referring to tossing something into a trash can, as in "what you're saying is garbage so shut up."
The English word 'lily' means any liliaceous plant of the genus Lilium having showy pendulous flowers
in on it means you know whats going on or about a certain subject."when it comes to penguins I am in on it!" i might say
I think it means that that person agrees with that others persons idiom and that it fit that question that the teacher or whoever asked that question.