What it means is if the enemy unit just passes their leadership test they lose one model.
For example, Space Marines lose say 2 models out of 5. They have to make a morale phase roll and roll a 6. With 2 casualties plus the morale roll of 6 that matches their leadership of 8, which normally would mean no losses from morale. With Tormentors they would instead lose a model. As an extra bonus, as they didn't fail the roll, I assume the Marines wouldn't be able to reroll either.