Invented by Melvil Dewey, the Dewey Decimal System is how you find a book, DVD, or other item on the shelf.
The official definition is:
The Dewey Decimal Classification (DDC) system is a general knowledge organization tool that is continuously revised to keep pace with knowledge. The DDC is published by OCLC Online Computer Library Center, Inc. OCLC owns all copyright rights in the Dewey Decimal Classification, and licenses the system for a variety of uses.
Think of it as the URL of a website. The call number takes you directly to the exact item you want. It will look something like this: 342.7302 A615