A parlay is a bet on two or more point spreads, over/unders or moneylines to increase the possible payout
The main reason to parlay bets is because the payout can turn into much more money than a single bet. For example, if you really like two teams to win, you can combine their moneylines into a parlay bet. Instead of betting one team at -150 to win and the other at -120, you could combine them. In a parlay, those odds would turn into +205, which means a $100 bet would pay out $305. If you bet $50 on each of those moneyline odds, the payout would be about $174.
The payout can grow as much as you want it if you keep adding bets, as a lot of sportsbooks allow up to 10-bet parlays or even up to 15. Of course, the more bets you add, the likelier it is the parlay doesn't succeed.
The problem with something like a 10-team parlay is that even if nine of those bets win, it doesn't matter if the 10th one loses. In order to win a parlay, all bets need to win (or push).
While you want every bet in a parlay to win, it's also possible that some of the bets push and you still earn some money. If you have three bets and one of them pushes, the parlay would turn into two bets, assuming those two are winners. Of note, not every sportsbook abides by these rules, though that's the most common method.