The main function of the CPU (Central Processing Unit) in a computer system is to execute instructions from programs by performing basic arithmetic, logic, control, and input/output operations. It acts as the brain of the computer, coordinating the activities of all other hardware components. The CPU interprets and processes various data types, executing operations at incredible speeds which determine the overall performance of the computer. Modern CPUs often have multiple cores, allowing for parallel processing to enhance efficiency in executing multiple tasks simultaneously.