Details about HEARTS
Hearts is a Super Rare Boost.
Boosts give you the ability to fly with your Battle-Car and make freestyles and aerial goals. It depends on your Battle-Car how many boost trails your car has. You can equip them inside the customization window in the rocket boost tab.
To get the Hearts Boost you can either open a specific crate with a key or trade with other players.
You can search for trade offers with Hearts here.