ECU (Electronic Control Unit), also known as "trip computer" and "vehicle computer". In terms of use, it is a special microcomputer controller for automobiles. Like ordinary computers, it consists of microprocessor (CPU), memory (ROM, RAM), input/output interface (I/O), analog-to-digital converter (A/D) and large-scale integrated circuits such as shaping and driving. To describe it in a simple sentence is "ECU is the brain of a car".
Modifying ECU is to change the engine operation by changing the method of dealing with problems (original ECU program). The so-called "ECU program" is actually a set of algorithms. It is stored in the memory, and the signal converted from the input device by the controller is processed to generate a corresponding command signal and transmitted from the output device. So modifying ECU parameters is actually modifying the algorithm.