Inside the majority of modern cars is an engine management system which is effectively a computer chip. This chip is connected to virtually all components of the car and is used to tell the engine how much power to apply, what mix of fuel and air to use, and even when to apply the turbo and to what extent. You can’t alter the diesel chip in your car yourself, but you can have it done by a professional tuner who can greatly increase your car’s performance in a variety of ways.
When a manufacturer creates a new model of car they install an engine management system and they use the same chip data in every single one. They must allow for people using different fuels and oils, different parts, different tyre pressures, and driving in different conditions. As such, your diesel chip is not personalised to you but to the average person.
Remapping of your diesel chip changes this. It replaces the stock data used by the manufacturer with data that is specific to your driving style and driving conditions. It provides additional power in the shape of BHP and torque, and it improves your mpg fuel economy figures too.