So far it is a normal type event exclusive move, exclusive only to the Celebi you can obtain from Pokémon Bank. It is basically a variation of the move False Swipe.
Hold Back inflicts damage, but will leave the target with 1 HP if it would otherwise cause it to faint. If the target has 1 HP remaining, Hold Back will hit and leave the target at 1 HP.
If used against a substitute, Hold Back is able to break it and will not leave the substitute with 1 HP.
Hope I helped :)