OverByter
Resident Slide Rule Guru
S-Off is something exclusive to HTC phones. Basically once you unlock the bootloader at htcdev it allows you to flash recoveries and roms without issue, however having s-off which stands for security off allows you to flash unsigned files to your bootloader and install kernels through recovery and also change your radio file. When you first root the phone it's basically only half unlocked, with s-off the phone is completely unlocked allowing you to flash any partition with unsigned files. Just remember that with greater flexibility comes greater responsibility because you are now responsible for everything that you flash, there's no longer a safely net protecting you from yourself so get in the habit of reading everything twice before you start flashing away. Sorry for being so long winded, gotta chill with the 5 hour energy. 



