I know these things can get a bit jargony, so a quick recap: to “jailbreak” means to modify a device to run code and applications not signed or approved by Apple, thereby allowing you to do things with your device far outside of what would normally be possible. “Untethered” means that once it’s jailbroken, it stays jailbroken (whereas a “tethered” jailbreak means the device resets to its normal, un-jailbroken state whenever it is reset)
The team behind this hack, Chronic Dev, is the same group that makes the greenpois0n tool that’s been jailbreaking iOS devices for years. Remember comex, the iOS hacker who went legit with an internship at Apple? He was a key member of this group.
While their server seems to be taking a bit of a pounding right now, you can find the new iPhone 4s/iPad 2 jailbreaking tool (dubbed “Greenpos0n Absinthe”) right over here.