Let’s try to stay with me here.
Microsoft released a service pack for Windows Vista because Vista kind of sucked as it shipped. The service pack, SP1, made many people’s Vista installs reboot permanently. So then it released an SSU (Servicing Stack Update) that Vista users were supposed to install before the SP1. This caused many people even more problems. So Microsoft today has announced a second patch, a pre-SSU update, that is to be installed before the SSU that is to be installed before SP1.
In other words Microsoft has issued a patch for its patch of its patch for its flagship OS.