What exactly is a "guarantee" when booking a cabin? What are the advantages and/or disadvantages? Is this something you have to request when booking? Thanks in advance for any feedback.

A "guarantee" booking or a TBA (to be assigned) bookingis basically paying a price for a certain category of cabin with the chance that you may be upgraded. If you don't HAVE to have a specific cabin, you can book a guarantee (normally at a cheaper rate) for a certain level (say a balcony or oceanview) and you are guaranteed at least that category of a cabin. Then if the cruise is not full or you book early enough, they will upgrade you into higher category cabin of the same class (like a larger balcony cabin etc..)

We have done it a couple of times. We have always seemed to get at least a little upgrade.
If you are not that fussy about your cabin it works out. Just be sure to book at the lowest category you would be happy with in case you do not get the upgrade.