Open Source
The term "open source" can mean quite a lot of different things to different people, but a short, quick explanation is that it is something that you can not only use as it is, but also look at the parts and create something new yourself with it. For example, you can not only download and play 0 A.D. you can also download the code and take a look at how the game is created, as well as edit it yourself if you know how to program.
From the Wildfire Wiki