Cosmati is a special type of decoration that you might find on floors or walls in really old buildings, like churches or palaces.
Imagine you have a big, flat piece of paper, and you want to make it look pretty. You could draw lines all over it, or make little patterns with different colors. That's kind of like what people did with Cosmati, but instead of paper, they used stone, marble, and other types of rock.
First, they would cut little pieces of rock into different shapes, like squares, triangles, or circles. Then, they would lay out those shapes in a pattern, like a puzzle. Once they had the pattern they wanted, they would fill in the spaces between the rocks with cement, or another type of glue, to hold everything in place.
The really cool thing about Cosmati is that sometimes they would use different colored rocks, to create pictures or designs. For example, they might use white rocks to make a swirl, or black rocks to make a letter, or red rocks to make a flower.
So when you look at a floor or wall with Cosmati on it, it's kind of like looking at a really fancy puzzle with different colors and shapes all put together to make something beautiful.