If you hear someone is adding fuel to the fire it means that they are causing a conflict or tense situation to worsen, making it even more tense than it already was. This is a common and straightforward idiom and it is usually used when someone instigates further conflict through provocative statements – even if these statements were not meant to be inflammatory.
To add fuel to the fire is quite transparent in its non-figurative meaning, since it is easy to picture a fire gaining intensity if you were to add additional fuel to it: for example, in the German and French versions of this idiom, you would add oil to the fire.
A good and topical example of its figurative meaning would be that the continued downplaying of police brutality by some elected officials in the United States is only adding fuel to the fire of the indignation behind the ongoing protests. The escalation in violence from the side of the police towards the protesters is another example of adding fuel to an already raging fire.