A parlay bet is a bet involving two or more separate selections, all of which must come in for the bet to win. These selections can be on the same game or event, or multiple different ones, and can feature different betting markets including the moneyline, point spread, total or over/under, or prop bets.
The main advantage to parlay betting is that it can massively boost your payouts compared to betting on each selection as individual bets. However, as you are relying on more than one outcome to happen, they are naturally harder to win.
Parlay odds are calculated by multiplying the odds of each selection within it. For instance, if you placed a $100 parlay bet with two selections both priced at -100 (a payout of 2:1), the payout would be $400 (2x2:1).
Some sportsbooks, such as FanDuel, exhibit their most popular parlays that day, so that you can be aware of where other bettors feel there is an opportunity for a big payout.