Apple has patched OS X against a security vulnerability called "Shellshock."
Shellshock is a bug discovered last week that, when exploited, can allow a remote user to control one or more computers in a system.
Once the computers are under remote control, they can be used for malicious purpose, like attacking websites with distributed denial of service (DD0S) attacks.
Apple has released a patch for OS X Mavericks users, as well as update for Lion and Mountain Lion users.
Apple claims only a limited number of users are vulnerable to the bug, but this is an update you should get now to rid yourself of any worrying.