The update brings the EFI for the late 2013 iMacs to version 2.1. It is aimed at addressing an issue sometimes occurring when installing Windows 7 or Windows 8 using Boot Camp Assistant. If both an external optical drive and USB thumb drive are connected to the system, the installation process could cause a black screen to appear.
Additionally, the update makes sure that a system will boot by default into OS X after installing Windows 8.
iMac (late 2013) EFI Update 2.1 is available as a free download from Apple's support pages. It requires a Mac running OS X Mountain Lion v10.8.4 (12E4022) or later. It is an 8.7MB download.