A sky lantern is like a really cool floating balloon that you can light up and it will glow in the dark. It's usually made of thin paper or fabric that is shaped like a hot air balloon with a small opening at the bottom.
First, you need to light a small fuel source, which is usually a thin piece of wax or string that burns slowly like a candle. You then attach this to the bottom of the lantern and wait for it to start heating up the air inside the lantern.
As the air heats up, it starts to rise, and because the lantern is so lightweight, it'll start to float up into the sky. Once it reaches a certain height, you can let it go and watch it float away, glowing in the dark.
It's important to be careful when using sky lanterns because they can be a fire hazard if they're not handled properly. You should only use them in open spaces away from buildings or trees, and never use them if it's windy outside. Once the lantern has floated away, it'll eventually cool off and come back down to the ground on its own, but make sure you don't leave any rubbish behind!