Overview
Source code for free software is typically distributed in compressed tar archives called tarballs. Debian is a binary-oriented distribution, meaning that its deb packages include precompiled binaries and data files arranged into a file system hierarchy that the software expects. The Debian build toolchain thus needs instructions on how to use the upstream build system to build correct deb packages.
These instructions are stored in the debian subdirectory, which is added to the source tree for the software being packaged by the package maintainer. While it is possible to build the package directly from the modified source tree, it is standard practice to create source packages, which contain the changes the maintainer made to the upstream sources in redistributable form.
Read more about this topic: Debian Build Toolchain