Okay kiddo, have you ever seen a ship on the water? It's really big, right?
Now imagine that ship is on fire. That's what a fireship is. It's a ship that is intentionally set on fire and sent towards other ships in order to cause damage.
But why would anyone do that? Well, a long time ago, before we had fancy guns to fight with, ships would get close to each other and men would climb onboard to fight with swords and other weapons. Fireships would be sent towards the enemy ships to scare them and cause chaos.
These days, fireships aren't used very often because we have other ways to fight. But they are still talked about in history books and movies.
So there you have it, kiddo. A fireship is a ship that's on fire and is used as a weapon. But don't worry, it's not something you need to be scared of in real life.