ECU remapping, tuning and chipping are terms that are used to describe the same thing. Both car and van engines have an electronic system that control fuelling and ignition. This is the ECU (Engine Control Unit).
When you put your foot on the accelerator, the ECU decides how much fuel to inject into the cylinder and when to ignite it. The ECU will also control the boost of the turbo (if you have one). Diesel and petrol engines work a bit differently but that isn't really important for the purposes of the ECU remap.
If you have your engine remapped, chipped, chip tuned or whatever other term you want to use then the ECU parameters have been changed to give increased performance and in certain cases more economy.
This can be done in several different ways, the most common is to connect to the vehicle diagnostic port and reflash the software on the ECU, with a standard map for the vehicle in question. This does require specialist equipment, it's not a DIY job, but as it is all done with software. Then a new map can be uploaded to your vehicle ECU pretty quickly and can often be completed in less than an hour.