Calibrations are the dataset that the ECU uses to run the engine. The tuner will generally setup all the data in a calibration therefore "matching" the ECU to your engines needs. Always be sure to start with one of our basemaps (or one that is as close as possible, or get in touch so we can generate you one) as there are ALOT of settings in an ECU and a missed option such as a limiter etc could stop the engine working correctly.