picture a high performance car as runner if you will... while a runner is speeding down the track he/she needs to breathe in and out, oxygen goes into the lungs and is exhaled as CO2...
Now while car's engine is running it also needs to breathe...cold air is taken in through the intake, mixed with gasoline, combusted, and your car goes! Most factory intakes and exhausts are restricting, thus not letting the car breath in and out to its top potential...so by switching some parts i.e. CAI, Headers, Exhaust pipes, SuperChargers, your car runs cooler, better, and faster!
i hope that somewhat explains it!