Most vehicles contain an ECU which is effectively a small computer that controls how the engine works. The manufacturer of these vehicles detunes the engine by setting the software on the electronic control unit to a default setting before sending them out to the car dealers. This is done as a result of the manufacturers having to sell their cars all over the world; this means that the software settings on the ECU must consider various climates, laws & regulations and varying quality of fuels. Vehicle remapping is basically the modification/replacement of the manufactures default software on a vehicles ECU.