Gambling is the wagering of money or something of value on an event with an uncertain outcome, with the intent to win money or physical items. Las Vegas, Nevada was a well known city that had legalized gambling.
In 1955, Biff Tannen's grandmother would make him drive her to Las Vegas to play slot machines, which took a nickel and had the potential to pay many times that amount if the three pictures on the spinning wheels lined up.
The next day, Biff drove his grandmother to Las Vegas. He was repeatedly trying to get his grandmother to bet for him at the Turf Club since the legal age to gamble was 21, and at 18, he was underage. He gave her a $20 bill that he stole from a table to his grandmother to bet on the Chicago Bears. She didn't bet on the Bears, and they lost against the Los Angeles Rams, making Biff twenty dollars poorer. Biff told her to make another bet, on the Washington Redskins. However, when they beat the San Francisco 49ers, she only gave Biff back his twenty dollars, and kept the rest for herself. When Gertrude claimed Biff was as greedy as his father, Biff stated that he learned that from her.
Lou noticed the altercation, and noticed that Biff used a book to select the winners. He wanted to get a look at Biff's methods himself. He told Biff that they could earn money together, as he would be able to make the bets while Biff could not. Biff reluctantly agreed when Lou told him to wait for him in the parking lot. While Biff was waiting, Lou killed Gertrude, and put her in the back of his car to bury her. He told Biff that he had talked to his grandmother and said that she was still playing the slot machines.
Lou stole the Grays Sports Almanac from Biff, and when he realized that it was the book that stated the winners, and not any methods that Biff had employed himself, he planned to kill Biff and bury him next to his grandmother. However, when he went to hit Biff with a tire iron, Biff caught it and killed him instead.
Biff justified the murder in his mind based on the fact that Lou had killed his grandmother. He buried Lou, and took all of the money that was in his briefcase. With the money, he covered up the murder of Gertrude by paying to have a death certificate forged that stated she died of natural causes.
Three years later, on March 28, 1958, his twenty-first birthday, Biff used the almanac to bet on horse racing and won his first million. In 1959, he gambled and won several more times, earning the nickname the "luckiest man on Earth".
In 2015, in the timeline where Marty posed as his son to keep him from going to prison, Terry stated that he wished he could go back in time to bet on the Chicago Cubs, who had just won the world series. Marty liked that idea, and went to an antique store to purchase the Grays Sports Almanac. Doc Brown discovered it, and tossed it in the trash, explaining that he didn't invent the DeLorean time machine for gambling. Biff overheard this, retrieved the almanac from the trash, stole the time machine, and traveled back to 1955 to make himself wealthy.