It means you have earned a free night (look for the category it specifies) either from a promotion or through use of a Marriott Rewards VISA Card. If you'd like to use it, find a hotel in that category and call the number on the back of your Marriott (not VISA) card. ProfChiara
I had an unused certificate and some how Marriott had made an error. Instead of using my certificate, they had deleted points from my account! It looks like they will fix the problem. When you get a bonus certificate, you must check out the highest category number on it...they are usually a '4' or a '5' which limits you to the hotel that it can be used for....still, it's a nice bonus if you can use it within in the time frame specified on it.