Everyone likely to read this knows that root can unlock a lot of doors for Android modding, and an unlocked bootloader opens even more. But for HTC phones, S-OFF is the ultimate in control, allowing users essentially unlimited access to every piece of code on the device. The Revolutionary dev team has released a new S-OFF method for the HTC One. You may remember these guys from exploits on most of the major HTC phones from 2011 and earlier.
S-OFF refers to a signature check and write access security in locked HTC phones. When it's on, the phone checks for signatures on certain portions of the software - the radio, recovery, certain parts of the system drive, and a few others - and keeps the user from writing to restricted areas.