Ok so The Great Gatsby is a book by F. Scott Fitzgerald. It's a book set in the 1920s, New York where there's a young man named Jay Gatsby who, five years before, had fallen in love with a woman named Daisy. When WW1 began, Gatsby left for war and left Daisy behind. Daisy found a man named Tom Buchanan in those five years and on the day of their wedding, Daisy received a letter from Gatsby explaining why during the war he'd never sent a card or anything. Nonetheless, Daisy and Tom got married. Gatsby found out and bought a house right across from her's, across the bay. Every saturday night he'd throw huge parties where the entire city would go; he'd do it in hopes that Daisy would wander in to his home and they could meet again. She never does. So when Gatsby finds out his poor little neighbor, Nick, is cousins with Daisy he asks Nick to invite Daisy to his house so that Gatsby could show up "unexpectedly" .
And that's all I can tell you so I don't give spoilers
It's also a movie! And they're both absolutely amazing!