Your gear will determine how many cards you will play. The car has four gears and you can move up or down one gear each turn. If you move two gears, you'll have to take a heat card into your discard pile. But it's up to you which cards you want to play each turn. You can't discard your stress and heat cards, so you'll eventually have to either play your stress (which means you'll play the top card from your draw pile) or shift to 1st or 2nd gear to cool down.
Just like if you're driving a car around a tight turn, you'll have to slow down, or you could spin out. You also have to make sure not to overheat, as this will cause more stress to your car.
While playing Heat, there will be a few corner checkpoints along the way. Below you can see a circle with a 3 and a circle with a 2. Your total speed while passing those has to be less than or equal to that number, or you will have to add heat to your car. You only get a certain number of heat cards (usually 6), so you have to be careful not to deplete your pile or you won't be able to complete the turn AND you'll add stress to your car.
There are a few other minor rules that I will let you explore on your own. It's really not too bad! It's categorized as medium light.