Sheryl,
A guarantee booking is a booking where you do not receive your cabin assignment until day of departure. For example, you can book a guarantee category 6A. This means you will receive at LEAST a 6A cabin, but you MAY receive an upgrade to a higher category. Keep in mind that if you insist on having a cabin midship or on a certain deck, stay with an assigned cabin. But if you are flexible and don't mind where your cabin is, it's generally a better deal since guarantees are typically a little bit cheaper than booking with a cabin assignment.