Basically the engine is controlled by the ecu, it has a standard map from the factory which gathers the information from the various sensors and this controls how much fuel is injected, how much boost the turbo has etc etc. (this is done every 1,000th of a second and with hundreds of different parameters). When the car is remapped the computer program for the ecu is rewritten so that you can have more power, economy or both if it's done right! Manufacturers tend to play safe and have to adhere to emission controls of various countries where the vehicles will be sold. Remapping opens up your engine to it's full potential. There are lots of people who claim to be the best, never take their word, always go somewhere that has been recommended. A bad remap can wreck your car, a good one will give you a big grin and years of happy motoring!