A guarantee cabin means you book a category rather than a specific cabin, and the cruise line assigns the actual stateroom later, sometimes not until days before sailing.
It is usually cheaper, and you may be assigned something better than you paid for. You may also end up next to the elevator on a low deck.
For price drop purposes the important part is that guarantee bookings are excluded from price protection on most lines, because there is no specific cabin to match a fare against.
