Can somone please explain what subversion is, what it does and how to use it?
Dude, that question was already answered in the other post of yours (http://silverstripe.org/general-questions/show/262213). I think the [url=http://en.wikipedia.org/wiki/Subversion_(software)]Wikipedia Article[/url] explains it quite in detail. What about searching for the answer yourself? This is by no means related to SilverStripe and doesn't need to be answered in this forum.
Perhaps we should add a glossary page to the wiki? Big Bang - you can see our SVN (subversion) notes here http://doc.silverstripe.com/doku.php?id=subversion though you are better starting on some of the information on wikipedia and on http://svnbook.red-bean.com/