Definition
In general, open source refers to any program whose source code is made available for use or modification as users or other developers see fit. Open source software is usually developed as a public collaboration between communities of developers and made freely available.
There's no such thing as a free lunch...
Well, this might be one of the exceptions to that old saying. However, increasingly developers are including their Open-Source activities on their CVs as proof that they are passionate about what they do, to the extent of giving up free time to collaborate with others. Of course, they also benefit because they don't always have to 'invent the wheel' when programming - someone, somewhere may have done it already, and is willing to share!