Apple had released iOS 4.3.4 to fix the vulnerability used by JailbreakMe 3.0, which was one of the easiest solutions to jailbreak iPhone, iPad and iPod Touch. Comex had also released a patch called PDF Patcher 2 on Cydia to fix the vulnerability associated with viewing malicious PDF files.
It was meant for users you didn’t want to upgrade to iOS 4.3.4, but also didn’t want to be exposed to the security vulnerability.