Open Source in terms of software means, that source code of a system is provided and you can access and have a look at it, so you can for instance learn how it works from the inside or you can extend it. Today Open Source has become mainstream (Firefox, Apache, loads of frameworks, etc.) not only in private non-commercial use merely in daily practical commercial software engineering. I learned quite a lot from Open Source not only from the software itself but from the contributing people who are giving their wisdom to the world by providing support and helping in forums or alike.