Imagine you have a lot of toys and you want to put them in groups so you can easily find them when you want to play with them. You might put all the dolls together or all the cars together.
The Library of Congress Classification is a system like that, but for books in a library. It helps librarians find and organize books based on what they are about.
The system has many different categories or groups. Each book is given a specific call number that helps it fit into the right group. For example, if a book is about animals, it might be in the group called "Q" which is all about science. And if it's specifically about dogs, it might have the call number "QL725."
This way when you go to the library looking for a book about dogs, the librarian can easily find all the books with the call number “QL725.”
It's important to have a system like this because libraries have so many books that it would be really hard to find what you are looking for without help. And the Library of Congress Classification helps us all find the books we need!