What the heck does that mean? The vulnerability can only be exploited if your copy of Norton scans a RAR file that has been crafted in such a way as to trigger a buffer overload. The update makes sure that your copy of Norton can detect these naughty files without falling prey to their naughtiness, for lack of a better word.
They have also posted instructions on how to setup your anti-virus software to skip over scanning .rar files. Definitely worth checking out if you are running any Symantec anti-virus products.