The function of the CPU (Central Processing Unit) in a computer system is to perform the majority of the processing tasks required for executing programs. Often referred to as the "brain" of the computer, the CPU interprets and carries out instructions from software through a series of arithmetic and logical operations. It manages data flow between various components like RAM, storage, and input/output devices. The performance of a CPU directly affects the overall speed and efficiency of the system, making it a critical component in computing.