It means security off. It allows the phone to be truely unlocked. You can then convert to GPe, or developer edition. Lots of benefits but be careful, with great power comes great responsibility.
Sent from my HTC One_M8 using XDA Premium 4 mobile app
AT&T HTC One (M8) - unlocked/s-off/rooted stock
Asus Transformer TF701T - unlocked/rooted stock