Bets is a game mode that allows players to bet their parts on other players' fights.
To place a bet, select the car that you think will win the fight and drag the parts that you want to bet on it. You can bet any part you want, including parts that are already in use. But you can't bet your last chassis and your last weapon. You also can't bet toolboxes or stickers. The details that you used as a bet cannot be sold or fused.
The number of slots available for your bets depends on the type of a bet and the skills you have unlocked.
To see the results, tap the "Watch" button when the timer ends. 
If your bet wins, the parts at stake will receive a random Magic Bonus. If you lose a bet, you will lose a part at stake. You can get it back by spending gems, but only right when the bet is lost - you won't be able to buy it back after. The longer the bet lasts, the bigger the bonus that you'll get. 
One part can only have one bonus. If you bet a Super Part (the part that already has a Magic Bonus), it won't get a new bonus. Instead, its current bonus will increase. Note that you can't gain more than a 60% bonus in bets.