Since the events of the movie take place about 70 years before Harry Potter comes of age as a wizard, Fantastic Beasts And Where To Find Them predates the births of most characters from the original franchise. There are two major exceptions: first, Albus Dumbledore, former Hogwarts headmaster; and second, Gellert Grindelwald, an ambitious wizard and Dumbledore's best childhood friend. In an interview with The Leaky Cauldron, Fantastic Beasts director David Yates confirmed that (spoiler alert) Johnny Depp would be playing Grindelwald in the new series. There has been fan outcry over the casting of Depp due to recent allegations of domestic violence, especially since Grindelwald's role in this franchise is shaping up to be quite important. Maybe it's been a while since you read the books and you can't quite remember his arc and its significance. Read on for a quick refresher of what happens to Grindelwald in the Harry Potter books.
Grindelwald's story unfolds to the reader as Harry begins to learn more about his headmaster and mentor. Dumbledore's character is inextricably linked to Grindelwald's, and these revelations show the young wizard that Dumbledore was never perfect, even if he did his best to make amends as he grew older. Grindelwald, meanwhile, chose a different path.
"For The Greater Good"
Grindelwald attended wizarding school at Durmstrang, but was expelled for playing around with the Dark Arts. (And Durmstrang is known for being lax about that magic, but using it on other students was way out of line.) He moved to Godric's Hollow to live with his great-aunt Bathilda Bagshot and met the Dumbledore family: Albus, and his siblings Aberforth and Ariana. Albus and Grindelwald grew close because of their shared beliefs and Albus's unrequited feelings for his friend. The boys believed that wizards were meant to peacefully rule muggles instead of keeping their abilities a secret from them. To achieve this new world order, Albus and Grindelwald thought they'd need to find and wield all three Deathly Hallows. "For the greater good," was their motto.
Aberforth objected to his brother's plans. An argument with Grindelwald led to Grindelwald using the Cruciatus curse on him. Albus tried to save his brother and in the melee, Ariana was killed. Grindelwald fled after that, possibly in guilt.
The Elder Wand
Albus Dumbledore's motives for seeking power were related to his family; Grindelwald's were more sinister. Without Dumbledore to check him, Grindelwald went full dark wizard. He stole the Elder Wand from a wand maker who was studying it and continued to use dark magic to achieve his goals. In 1945 (a purposely symbolic year, per J.K. Rowling), Albus challenged his old friend to a duel and claimed the Elder Wand for himself. Grindelwald was sentenced to a lifetime in Nurmengard prison. He died there when he refused to give up Dumbledore's grave as the location of the wand to Voldemort.
To recap: Ariana's death and Grindelwald's exile begin in 1899. The dark wizard's defeat doesn't occur until 1945. Rowling hasn't announced how many years the Fantastic Beasts series will encompass, but it seems that the action will take place mainly during Grindelwald's active period, which means that these movies have the potential to fill in many blanks in the life of Gellert Grindelwald as it exists in canon so far.