What exactly is Guava Island? With the film shooting in secret on Cuba and only a vague teaser for promotion, it’s hard to say. But here’s what we do know: it stars Donald Glover and Rihanna, and it’s directed by quickly rising star Hiro Murai (“This Is America,” Atlanta, Barry). Also, now it’s coming out for free on Amazon Prime and Twitch beginning on Saturday. Watch it this weekend, or be completely out of the loop when we’re all talking about the socio-political meaning of guavas next Monday.