Definition of must noun from the Oxford Advanced Learner's Dictionary

BrE BrE//mʌst//
 
; NAmE NAmE//mʌst//
 
[usually singular] (informal) something that you must do, see, buy, etc. His new novel is a must for all lovers of crime fiction. Trips to Pompeii and Naples are absolute musts.
Word Origin Old English mōste, past tense of mōt ‘may’, of Germanic origin; related to Dutch moeten and German müssen.
See the Oxford Advanced American Dictionary entry: must