You only have a limited amount of fuel for each drive. What you need to do is do as many back flips and front flips as you can to earn money. Also, you need to land your flips or you will die before your fuel runs out and you won't earn as much money. When you earn a certain amount of money then certain upgrades will become available to you. As you progress you will also earn more fuel, but you don't have to pay for that part. See how many flips you can do, how far you can go, and how awesome you can make your car with all the upgrades. You can eventually unlock different vehicles as well.