'Open Source’ originated in the software development field. If something is open source, it is freely available to use, and also to modify. It has also come to encapsulate an ethos of collaboration, participation, community idea exchange, and meritocracy. Projects are often worked on by widely dispersed communities, working to develop or improve a product, or fix issues. Open source is generally altruistic (most products or initiatives are free to use).
Further reading: http://opensource.com/resources/what-open-source