How would you explain the statement "Daisy tumbled short of his dream"?
Nick makes that observation at the end of the evening when Gatsby and Daisy are reunited for the first time in five years (Chapter 5).
Gatsby and Daisy were in love years ago, but she married Tom. Gatsby, apparently, has spent the entire five years trying to get back to Daisy. He "earned" the money to buy a mansion across the water from hers, and he has been throwing lavish parties, hoping she would come to one. And all this time, he has been looking over at Daisy's house, and thinking of Daisy, and imagining what it would be like when they finally meet again. In his mind he has built up this moment to an impossible height.
It is this dream that (Nick suspects) Daisy fell short of a few times that evening. The actual sentence is, "There must have been moments even that afternoon when Daisy tumbled short of his dreams -- not through her own fault, but because of the colossal vitality of his illusion." His dreams are the way he has remembered Daisy -- her beauty, her personality, her love for him, and so on -- and the way he has imagined their reunion would go. After five years of embroidering these dreams, no actual person, however lovely, could live up to them perfectly.