A forge is like a special oven that is used to melt metal so that it can be shaped into cool things like swords, horseshoes, and other metal objects. It's sort of like a giant toaster oven, but instead of heating up bread, it heats up metal.
People who work with metal use a forge to heat up the metal until it becomes extremely hot and soft. They then use special tools to shape the metal while it's still hot, like how play dough can be shaped when it's warm. Once the metal cools down, it keeps its new shape and becomes very hard and strong.
A forge usually has a big, fiery flame inside that gets really hot. This flame is usually made from coal or propane gas, which makes the forge super hot. People who use a forge have to be very careful because the forge can get so hot that it can even melt other things around it, like the floor or the walls of the workshop.
So, in summary, a forge is a special oven that is used to melt metal so that it can be shaped into different cool things, and it has a really hot flame that can make the metal super hot and malleable.