Backstab is a thief skill where you sneak up on your opponent and stab them in the back with a dagger or other piercing weapon. After backstabbing, the attacker is lagged for a short time.
Note that backstab is not totally effective on many creatures. Skeletal creatures are nearly immune, while creatures that are injured (HP at 30% to 90%) are generally much more aware and harder to sneak up on. Creatures who hav 0-29% hp, or 91-100% hp, are far more likely to be successfully stabbed. In PK, injured targets (30-90% HP) have about a 50% chance of being backstabbed as compared to a far lower (About 6%) for mobs.