ECU remapping is taking a read from the ECU’s processing chip of the vehicles standard compromised map and adjusting various parameters within the map such as fuel pressure, boost pressure (on turbocharged applications) ignition advance and throttle pedal control amongst others to release the true performance from the engine. It is a completely safe process as it is just giving the engine the performance it should have had in the first place before all the compromises were applied to the original programming.