The Dewey decimal system is a numerical system used in organizing library books. Because books are, for several reasons, hard to organize by a simple linear system like 1-2-3.
The system of 1-2-3-etc would make enormous numbers because there are over a billion books and it leaves no room to sort the books by category. Melvil decided to make a system that allowed for sorting. First number was the main category, then another number signified the subcategory, etc This allowed for much shorter notations and easier navigation to the right book shelves when finding or shelving a book.