A futures bet is a bet on a future event, usually before a season starts. It's one of the easier terms to understand in sports betting because they deal with betting on the future.
Most sports have the ability to bet on futures because every sport has a future. In the NFL, you can bet on who will win the Super Bowl, who will play in the Super Bowl and who will win each division, among other things. In college football, you can bet on who will win the Heisman and who will win each conference. In soccer, you can bet on who will win the league and who will lead the league in scoring.
An example for Super Bowl odds would look something like this, though the list would go 32 teams deep:
New England Patriots
Kansas City Chiefs
San Francisco 49ers
Going further for NFL futures, you can also bet on how many touchdowns or yards a certain player will get. For example, you can make a futures bet on Tom Brady if he will throw for more than 4,500 yards or more than 30 touchdowns.
Not every sportsbook has an extravagant list for futures bets, but as sports betting grows across the country, the amount of things you can bet on before the season will only grow.