When you want to build up a campfire, barbecue or log fire in a fireplace, you need to add fuel to make it bigger. The fuel might be wood, charcoal or any other burnable material. The fire will then get hotter and larger, so if that's what you want, it's a good thing. On the other hand, if the fire is not under control, such as a forest fire, kitchen fire, or a house on fire, adding fuel to the fire would be stupid, because it makes a bad situation become even worse.

Sometimes in conversations, business or even social situations between friends, someone does or says something that upsets others. Then, instead of apologizing and making it better, that person may say or do something even worse. That kind of stupid action－making something worse that is already bad－is what we refer to as "adding fuel to the fire."