At CEDC, we have a bias towards free and open source software solutions in our web development processes, for a variety of reasons which are both practical and philosophical.
In practice, an open source project shares the source code that makes up the software and provides a license that allows modification and redistribution of the code without licensing fees. Most often there are communities of people that use the software and contribute improvements back to it, which are vetted, discussed, and often included in future versions for everyone to benefit from. Freely available open source projects power most of the web at this point.