The function of a CPU (Central Processing Unit) in a computer system is to act as the "brain," executing instructions and processing data. The CPU performs basic arithmetic, logic, control, and input/output operations specified by the instructions in a program. It interprets commands from software applications and coordinates the activities of other hardware components within the system. The speed and efficiency of the CPU significantly affect the overall performance of the computer, impacting everything from application responsiveness to multitasking capabilities.