CAI stands for Cold Air Induction. It is an intake tube that replaces your stock air-filter system. It does 2 things:1. It places the air inlet in a location that allows it to get colder air instead of the heated air in the engine compartment. Colder air is more dense and contains more oxygen. More oxygen means more power.2. It is less restrictive and allows in more air. Stock air boxes are designed to be quiet but in the process they become somewhat restrictive. A CAI will be louder but to some, that's a good thing.