This update fixes extremely important issues and also helps improve security. It includes fixes for vulnerabilities that an attacker can use to overwrite the contents of your computer’s memory with malicious code.
It is available via software update or the website.
There is a trojan out there that tricks PC’s and Macs into redirecting all their internet traffic through malicious servers. Even though the creators have been arrested, millions of PCs could still be infected.
Check out this article on Mashable to get more information.