Dewey Decimal System
The Dewey Decimal System is a way of library classification that is used internationally. It was invented in 1876 by Melvil Dewey. It is used to organise books in libraries so that they are easy to find and put back. It also makes it easier because every library uses it, so you only need to learn one classification system to be able to find books everywhere. Jem does not actually know what he is talking about when he mentions it in this context, he has most likely overheard adults discussing it and is using it to sound smart in front of Scout.