For the next 6 turns, whenever target champion would take damage from a ranged attack, that damage is dealt to the attacking unit instead. This effect is hidden until triggered.
PoxNora Release Set
Note if the attacker has an effect which is triggered by damage (such as Poison), those effects are done to the attacker if the attacker took damage from the attack. (In general, the attack proceeds as normal, the damage is just done to a different champion.)
Also note, if the target has righteous deflection and the attacker also has righteous deflection, the attack will continue as if both champions didn't have it.