Dubbed MS14-066, the patch shores up a hole that would have allowed an attacker to remotely trigger code on your machine using specially crafted packets.
At the moment, Microsoft says they don’t know of any exploits that have actually used this, but it’s still a good idea to get the patch installed before there’s a problem.
This isn’t too different from any other security patch, but the sheer number of machines affected means it’s good to make sure your system is up to date. You can grab the patch from Windows Update right now.
Via Life Hacker