That's ok - every market has their acronyms!
Wikipedia has a good definition. How it applies to software can be implemented in a few different ways, but at a 50K view it can be described as software (and the source from which it was built on) is available to the general public. Then, subject to the license agreements, recipients who acquire the code can modify the software to meet their particular needs.
Another good backgrounder is the Cathedral and Bazaar - which many point to as a manifesto about the differences between public open source efforts and commercial software development. (again - 50K: don't kill me on that one. )
There are almost religious debates over the benefits of open source software, some of which are summarized here. Some go as far as stating that using open source makes you an enemy of the state (i kid you not! )
In marketspeak, to opensource an effort means to make the software source available to the public (one can still charge for it) and then make money via support and / or customization services.
What it's not... Many companies that operate as a platform for applications, like Facebook or the iPhone, maintain total control and all rights over the platform. This usually means that to build on the platform you have to use a propriety format / programming language. And that what's allowed to be built on the platform is up to the platform owners.
Rather than consumers being able to install / us whatever applications they want on the platform, users of closed platforms can only choose applications the platform owners have approved, and that were developed specifically for that platform.
What it is... Opening a platform can actually mean several things:
Does that answer your question?