An OS (Operating Sytem) manages the hardware and networking. It copies my keyboard typing and displaying it on the monitor. The OS lets the system talk to the outside world in a secure way.
The OS manages the software. It reads and writes my files and directories on the disk. It runs many programs and assists many users at the same time, and makes sure they don't interfere with each other.
Debian is a project that creates free OS (Operating Sytem) distributions. I use it because it is very stable, tested, documented and supplied with applications. It is an open system using open licensing.