Okay kiddo, let me explain what a monopropellant is.
Imagine you have a toy car that runs on fuel. You need to fill up its tank with gas so it can move. Now, instead of a toy car, imagine a spaceship. Just like a car needs fuel, a spaceship also needs fuel to move in outer space.
Usually, spaceships use a combination of different fuels to propel themselves forward, like liquid hydrogen and liquid oxygen. But some spaceships use just one fuel, and that fuel is called a monopropellant.
Monopropellant is a special type of fuel that is made of just one substance. That substance is usually a type of chemical called hydrazine. It's a really strong fuel and can explode easily, so it needs to be handled carefully.
When the spaceship's engine burns the hydrazine, it releases a lot of energy, which pushes the spaceship forward. So basically, monopropellant is just one type of fuel that is used to make a spaceship move forward.
I hope that helps you understand, little one!