A guarantee means that you are guaranteed to bee placed in the category of cabin you booked.
In general, you pay the lowest price they offer for that category and they will upgrade IF they have a better obe available, but the rule of thumb is to never expect anything beetter than what you paid for - because most ships sell out (and we hear people saying that they were told "sorry, the ship is full" whenever they want or need to change cabins all the time).
But many people book guarantees (including me) because the difference between one balcony and the next is minimal (in my opinion).
For example - lots of ships charge more for the exact same cabin layout just because it is on a higher deck. I think this is the epitome of ridiculous - you can't tell what deck you are on when you are inside your cabin.
The difference between a balcony on deck 6 and deck 10 can be $200 per person or more, but the experience is not that different.