Well kiddo, intonaco is a special kind of plaster that's used on walls. When someone wants to decorate a wall, they first use regular plaster to create a smooth base. Then, they put intonaco on top of the regular plaster.
Intonaco is special because you can paint or draw on it really easily. People have been using it for a really long time, even as far back as ancient Egypt!
So basically, intonaco is like a special kind of paper that you can put on walls and draw or paint on. It helps people make beautiful decorations in homes, churches, and other buildings.