Off-hand means that the hand, which is originally equipped with a shield, now can be equipped with a one-hand weapon after you learn the off-hand skill. That is, you would have two weapons on two hands respectively, and have twice attacks. But, it should be noticed that some one-hand weapons can not be fitted in off-hand, such as main-hand weapons , and one-hand weapons showing "unique" in weapon status can not be equipped on both hands at the same time.

offhand is the secondary hand, the other hand, the one that it is not the dominant hand, usually the left hand, but, you know, not everyone is right handed, so maybe translating it as secondary hand is the best option