Having the stat Attack against Leather Armored Target Offsetting will reduce your damage taken by a percentage if you are wearing leather. This damage reduction is based on the level of the enemy attacking you. You can view this percentage for an enemy of equivalent level to you in your F1 stat menu.
Having the stat Attack against Leather Armored Target Offsetting will counteract the opposite Attack against Leather Armored Targets stat, which is a stat that increases damage dealt against enemies that are wearing leather. If an enemy has this attack stat, their % damage gain against you will be reduced based on how much offset you have.
If an enemy does not have this stat, then the offset stat will do nothing in PVP. It exists only to counteract the opposite attack stat. In the end, this can only ever reduce your damage taken back to the default amount if it was boosted by this opposite attack stat, and no further.