Most modern vehicles contain an ECU (or Engine Control Unit) that is effectively a small computer which controls how the engine works. Vehicle manufacturers de-tune the engine by setting the software on the ECU to default before sending the vehicle out. This is done due to the manufacturers having to sell their cars all over the world, this means that the software settings on the ECU must take into account different climates, laws & restrictions and varying quality of fuels. Vehicle remapping is basically the modification/replacement of the manufactures default software on a vehicles ECU.