RedSnow最近频繁升级，最新的版本是RedSnow 0.9.10 beta 3
From the Dev Team blog:
The b3 version of redsn0w fixes a problem where re-running redsn0w over an existing jailbreak would cause MobileSubstrate-based apps to stop running until MS was installed again. Now you can re-run the redsn0w jailbreak step without worrying about that (but still remember to de-select the “Install Cydia” option if it’s already installed).
What is Jailbreaking?
Jailbreaking is the process by which Apple’s mobile operating system, iOS, is modified to run unsigned code in order to gain access to files that Apple wouldn’t normally let you access.
Jailbreaking adds unofficial application installers to your iOS device, such as Cydia, which let you download many 3rd-party applications, tweaks, and extensions that are unavailable through the App Store. These packages open up endless possibilites to do things on your iOS device that a non-jailbroken iOS device would never be able to do.
You can install extensions that give you instant access to your system settings from anywhere on your iOS device, bypass certain restrictions set in place by Apple and the carriers, and find packages that give you more control over your iOS experience.
Jailbreaking is about freeing your iOS device from Apple’s restrictions to let you install anything you want.