A gatling gun is a type of gun that can shoot lots and lots of bullets very quickly, almost like a machine. It's named after the man who invented it, Richard Gatling!
Imagine you have a toy gun that can shoot one marble at a time. You have to put a marble in, cock the gun, and then pull the trigger to shoot the marble out. Now imagine that same gun but with 6 barrels that can spin around really fast. Instead of having to put one marble in at a time, you can put 6 in all at once!
When you pull the trigger, the barrels spin around and shoot out the marbles one after the other, really fast. And because there are 6 barrels, you can shoot 6 marbles before you have to reload the gun. This is how a gatling gun works, but with bullets instead of marbles.
Gatling guns were first used in battles a long time ago during the American Civil War, and they were very effective because they could shoot so many bullets in a short amount of time. Today, we have much more advanced guns, but the gatling gun was an important invention in the history of weapons.