Microsoft announced that a previously unknown security flaw involving fonts (of all things) could let a hacker take over your computer. This issue affects virtually ALL versions of Windows.
If your Windows computer updates automatically, you probably do not need to take action, as an update will be uploaded to your computer (hopefully before a hacker exploits the flaw!).
More information from Microsoft is available here: https://technet.microsoft.com/library/security/MS15-078