Okay kiddo, so you know how sometimes there are really big fires and it's hard for the firefighters to put them out? Well, imagine if there was a really, really big airplane that could help put out those fires. That's called a 747 supertanker!
This big airplane has a special tank inside of it that can hold a lot of water or a special kind of foam that helps put out fires. When it flies over a fire, it opens up a big hole in the back of the plane, and the water or foam falls out onto the fire to help put it out.
The reason the 747 supertanker is so special is because it can carry a lot more water or foam than a regular fire truck or helicopter, so it can put out bigger fires. It's like having a giant water balloon that can drop water on the fire to make it go away!
So that's what a 747 supertanker is, a really big airplane that helps put out big fires by dropping a lot of water or foam onto them. Pretty cool, huh?