SVN stands for "SubVersioN" ...no it doesn't mean much to me either

It allows the developers to submit changes to the code in such a way that changes can be easily monitored, quickly merged together with other changes. and (if necessary) reverted

It allows users to gain access to the latest (often "in-test") features

It is divided in to three chunks:
trunk - the main development & release code
branches - sometimes a developer may spend a week-or-more making their changes, so (s)he will work in a copy of the code until the changes are approved by the project leader ...then the changes are merged back in to trunk
tags - these are just copies of the code at critical points, such as official releases

To obtain the "cutting edge" codebase (ie. the latest in SVN) you need an SVN tool ...the same as: if you want to view a web page, you need a web browser

For Windows you will choose: TortoiseSVN
For Debian you will choose: apt-get install subversion
For others you will need to do a bit of research (cos I only use debian & windows)

Windows:
Create a directory to hold the code
Right click it and choose "svn checkout"
Enter the URL of the SVN 'repository':  http://grrlib.googlecode.com/svn/
Click the [...] button and choose the trunk
Leave advanced options alone (Ie. fully recursive, head)
Hit OK
If you network connection dies half-way just right click the directory again and choose "SVN Update"
You may also perform an update any time you like to get the latest & greatest code changes

Linux:
Read this google guide:  http://code.google.com/p/grrlib/source/checkout

BC