The handle in sports betting is the amount of money in wagers accepted over a specific period of time.
The most popular use of the term revolves around the Super Bowl which always has one of the biggest handles of the year. Sportsbooks often keep tabs of their handle for a specific event so they can compare to prior years, which is the case with the Super Bowl.
The handle can also be used by sportsbooks to help set betting lines because if a certain game or side receives a lot of attention at one time, something may have to be changed. If that happened, professional bettors would likely flock and bet a certain side because they noticed something incorrect with the original odds.
Even if there isn't a problem in the odds, the handle is always useful for bettors if they are keen on knowing where the money is going (public money) on a certain bet.