Clavis patrum graecorum is a big book full of important information about Greek Christian writings from a long, long time ago. Think of it like a magical key that opens up a door to the past, so we can learn more about what people thought and believed a really, really long time ago.
See, before the book was made, there were all these old writings that were really hard to find and read because they were written in a different language and some of them were very old and fragile. So, some very smart people decided to make a big book that would act like a map or an index, and tell us where to find all these old writings, what they were about, and when they were written.
The book is so big and important that it took a lot of people many years to make, and it has a lot of names and numbers in it that can be confusing at first. But, once you figure out how to use it, you can learn a lot about what people believed and thought a really, really long time ago.