It's a free, open source operating system for your computer. An operating system is a set of basic and auxiliary software that runs on your computer. Debian is more than a raw operating system. Debian comes with more than 51,000 packages, compiled software that is prepared in a very convenient format and easy to install on your system. One of the basic tenets of this project is that all products must be free. Free software is software that can be used freely, without restriction, for any purpose, study and review, and change.
It should be noted that Debian is a major distribution and many other Linux distributions have been adapted from it, such as Ubuntu.