According to Microsoft, the update fixes a vulnerability that could allow someone to overwrite the contents of a computer's memory with their own code. For complete information on the vulnerability and the fix in this update, look here and here.
The first issue, described in Microsoft Security Bulletin MS10-003, notes that an Office file containing code that exploits the vulnerability could gain full user rights to a machine when opened by a user, while the issue described in Microsoft Security Bulletin MS10-004 is specific to PowerPoint.
Note that this update requires Mac OS X 10.2.8 (Jaguar) or a later and the above Office products.
*I'm still using Office X, which was released way back in 2001, so don't feel badly.