Fermacell is like a really strong and sturdy piece of paper that people use to cover walls and ceilings. It's made out of a mixture of different things like gypsum, which is a white powder that's really good at holding things together, and paper fibers that help make it strong.
When people want to put fermacell on their walls or ceilings, they mix up some wet stuff called plaster just like a grown-up might make pancakes. They then spread the wet plaster on top of the fermacell and let it dry, kind of like how a grown-up might put syrup on top of their pancake to make it taste extra yummy.
Once the plaster dries, it hardens and becomes really strong, holding the fermacell in place and making it the perfect material for building walls and ceilings. Think of Fermacell like when you glue macaroni to a piece of paper and dry it, it gets hard and you cannot easily pull it off. That's how fermacell works, it sticks to walls and ceilings and stays there really well.