Other forms: added fuel to the fire; adding fuel to the fire; adds fuel to the fire
If two friends are arguing and you butt in trying to help, you might add fuel to the fire by saying something that only makes them angrier! Someone or something that makes things worse is adding fuel to the fire.
If something starts burning, and someone mistakenly pours oil on it instead of water, it'll only strengthen the fire! People and events can have a similar effect in a situation — for example, if one of those fighting friends wins a contest they both entered, it could make their fight worse. When using the idiom add fuel to the fire, you can insert more detail before or after fire, as in "The mayor's proposal added fuel to the political fire" or "Rising food prices added fuel to the fire of inflation."