Ok, kiddo! The Plan of Saint Gall is a very old and fancy drawing made by some clever monks back in the time when kings and queens ruled over countries.
You know how you have a room in your house where you play, a room where you sleep, and a room where you eat with your family? Well, the monks wanted to have a big building where they could do all their monk activities like praying, eating, sleeping, and studying.
So, they made this really cool drawing of a building with lots of different rooms, just like in a castle. Each room had a special purpose like a kitchen for cooking, a church for praying, and a big garden to grow food.
The funny thing is, this drawing was made a really long time ago, like over 1,000 years ago! But it's still really interesting to look at and learn about because it shows us how different people used to live back then.
So, that's the Plan of Saint Gall in a nutshell, little one. It's like a super old blueprint for a big building made by some smart monks a loooong time ago.