Pop open the App Store app as soon as possible—it’s time for another software update.
This particular update addresses a significant security flaw that affects the Network Time Protocol (NTP) component to OS X—basically, what lets your Mac keep its clock in sync with an online time server. The bug, according to Apple, “would have allowed an attacker to trigger buffer overflows,” possibly allowing attackers “to execute arbitrary code.”
In short, that’s security-speak for “attackers could run malware on your Mac.”
The OS X NTP security update offers a fix for OS X Mountain Lion (10.8.x), Mavericks (10.9.x), and Yosemite (10.10.x). Visit Apple’s website for the nitty gritty.
[Update: As Ars Technica points out, your Mac may have downloaded and installed the NTP Security Update for you. If you Mac automatically updated itself, you would have seen a notification banner telling you that “a new security update was installed on your Mac.”
If you’re running Yosemite and want to have future security updates installed automatically, go to System Preferences, click App Store, then check the box labelled “Automatically check for updates” if it isn’t already. Finally, check “Install system data files and security updates.”]