Unfortunately the patch notes don’t tell users what was wrong to begin with (presumably they don’t want hackers exploiting it and attacking those who have yet to patch their computers), but it is said to be a vulnerability that was discovered by google’s security team. Basically through the exploit, attackers are able to execute arbitrary code with the privileges of the ntpd process.
In any case the update should be waiting for you already if you were to launch your computer, and if you have been putting off on updating OS X, it certainly sounds like this is something you’ll want to get on right away. The update should be available via the Mac App Store in the updates section. Alternatively users could go to Software Update via the Apple Menu to check and download the update for their computer.