A "(fill in category) guarantee" means that you are guaranteed to get a cabin in the reserved category or a higher category. If you are upgraded to a higher category, you'll get the upgrade at no additional charge.
The advantages of booking a guaranteed category instead of specific cabin # are that (1) the cost is often lower than if you book a specific cabin and (2) you are more likely to get a free upgrade.
The disadvantage of booking a guaranteed category is that you might well get stuck in a cabin that is close to the bow where you'll feel the ship's motion much more than in a mid-ship cabin or you might wind up in a cabin with a dance floor or the ship's engines or some other noisy activity immediately above or below your cabin.
I always book a specific cabin after researching the ship's deck plan and checking the ship's reviews at
www.cruisecritic.com . As an example, I'm booked in a cabin for a January cruise on Holland America's Volendam that has a window and an adjacent balcony that is semi-private. (The balcony is open to everyone, but no one uses it except those whose cabins are next to the balcony.) But Holland America sells the cabin as a low-cost "inside" cabin. CruiseCritic buffs rave quietly about the cabin.