You’re going to want to update as soon as you can:
Available for: OS X Mountain Lion v10.8.5, OS X Mavericks v10.9.5, OS X Yosemite v10.10.1
Impact: A remote attacker may be able to execute arbitrary code
Description: Several issues existed in ntpd that would have allowed an attacker to trigger buffer overflows. These issues were addressed through improved error checking.
Scary stuff. Do this right away: it’s 1.4 MB, it requires no reboot, and it’s in the App Store. Just go do it.
Update: This is apparently no longer theoretical.