What is open source?
The dictionary definition of open source is:
denoting software for which the original source code is made freely available and may be redistributed and modified. | ||
--New Oxford American Dictionary |
While there are no license fees associated with Drupal, the most important part is the fact that you are free to modify and extend the code base to meet your own needs.
Open source means that you can also see the full history of the code—you can see when changes were made, for what reason, and by whom. There is no "black box" technology—everything is open for you to investigate if you desire to.