The IPv6 protocol allows remote attackers to cause a denial of service via crafted IPv6 type 0 route headers (IPV6_RTHDR_TYPE_0) that create network amplification between two routers.
Other fixes in this update include a patch for USB and bluetooth functionality, along with several generic “operating system fixes”. Additional application compatibility was included and adjustments for RAW cameras was added as well. The entire update package ranges from a 25MB download to a 391MB download depending on what you are running.
This is the sixth patch release Apple has crafted this year, with them more or less beginning to fall into a monthly patch cycle. The increased popularity of OS X, in no small part due to Apple's introduction of Intel-based Macs, likely has prompted them to develop more. At least their users would hope.
You can download the patches yourself at their site if you so wish.