A rotary kiln is like a big oven that spins around and has a long tube inside. Imagine a big hamster wheel, but instead of hamsters running around, there is a big tube for things to cook inside.
When you put things inside the rotary kiln, like rocks or minerals or even waste materials, they slowly move through the tube and get heated up by flames and gases that are blown through the tube. This makes them change in some way, like drying out or melting, in order to make whatever product is needed.
The kiln spins around slowly to make sure that everything inside gets heated evenly, like when you turn a rotisserie chicken around so that it cooks evenly.
Once everything is cooked or processed, it comes out the other end of the tube and is ready to be used for whatever it was made for.
So, a rotary kiln is basically a big spinning oven that is used to process materials using heat and gas in order to make products for various industries.