A licensing model for software which keeps the source code open and accessible. However, there is much more to Open Source as it's been used widely these days. The details of the software license depend on the specific Open Source license being used, e.g. a variety of GPL, MIT, Apache and others are available. Above and beyond that, Open Source invites developers to look at the code of others (open access), learn from the code of others, fork the code and develop their own branch, contribute back to the open source project of others and last but not least open and freely use and share the project by yourself or for others.
Drupal is one if not the largest Open Source software projects on the planet and as such a role model for what makes a successful Open Source project. It's not that everyone is just doing what they want, instead tight planning and management, quality assurance coding standards, security teams and a whole lot more is involved to keep and push Drupal at the edge of high end web frameworks.