A desideratum is something you desire or want. It's a fancy word for a must-have.
This official-sounding Latin word means something very common: anything you want or need. A new book could be a desideratum. A chocolate cake could be a desideratum. More commonly, a desideratum is something that is truly needed, like food or shelter. For many people, a happy marriage is a desideratum. For a politician, getting elected is a desideratum. You can almost see the word desire in desideratum, and that should help you remember it’s an object of desire.