A central-processing unit, or CPU, is essentially the brain of a computer. It's the main hardware component that performs most of the processing inside a computer. To understand its role, think of a CPU as the part that takes instructions from a program or application and performs calculations, runs processes, and manages data that allows the computer to function and run tasks.

Here's a simple analogy: If a computer was a human body, the CPU would be the brain, receiving information, making decisions and sending instructions to other parts of the body (or computer) to carry out tasks.

In technical terms, a CPU carries out basic arithmetic, logic, control and input/output (I/O) operations specified by the instructions in the program. Modern CPUs are typically composed of multiple cores, which are individual processing units within the CPU that can each work on different tasks simultaneously, making the computer faster at multitasking and processing complex programs.