Open source project gives you many freedoms with respect to the program. The most essential is a right to fork. E.g. change, re-use and distribute your own modifications to original program. Open source movement as a whole is in opposition against closed source - e.g. programs distributed with some kind of restrictions on end users or re-distributors.
There seem to be two main streams inside Open source:
- GPL like one - e.g. you have right to fork and your work needs to give right to fork to anyone else
- Apache, BSD, EPL (originally CPL), etc. - you have right to fork and you can do what you want with the result
NetBeans project falls into the second category and it helps the wide spread re-use of NetBeans Platform. On the other hand, GPL licensing also has its benefits as project Netbinox used to show (now re-licensed to the same license as NetBeans).