Idiom refers to a word, phrase or an expression whose collective meaning differs from their individual (words) meaning.
Once in a blue moon
Meaning: Something that occurs very rarely.
Ex.: Since my friend went abroad, we talk only once in a blue moon.
Add fuel to the fire
Meaning: To make a situation much worse than what it originally was.
Ex.: The Principal’s indifferent attitude towards the students’ grievances added fuel to the fire.