Windows has just advised me to install a "Cumulative Security Update for ActiveX Killbits." What are Killbits? How will I/my computer be harmed if I do not install a Killbit update?
So basically, a the KillBits list is a list of ActiveX components that will never be allowed to run on your system due to them having security holes. This list needs regular updates.
You could also check here for some more elaborate information about KillBits than Wikipedia offers: http://blogs.technet.com/b/srd/archive/2008/02/06/the-kill_2d00_bit-faq_3a00_-part-1-of-3.aspx
Killbits within ActiveX is a security feature. Basically if someone find a security hole in an ActiveX control/plugin microsoft can issue a Killbit that can disable the control/plugin.
Oops beaten to it, the Wikipedia article explains it much better than I have :D