Unique is related to bonuses. It means that if you redeem an offer that allows you to purchase multiple items in a single shopping trip, it will count as ONE total redemption towards a bonus. If you go shopping again and redeem the same offer, that redemption will again count towards the same bonus.
One way to tell if a unique offer is included in a bonus is if you look at the offer details and it says, "Limit 3 per receipt." This means you can redeem up to 3 products for that offer, which counts as one redemption towards a bonus, on a single shopping trip.