The CPU is the brain of a computer, containing all the circuitry needed to process input, store data, and output results.
The CPU is constantly following instructions of computer programs that tell it which data to process and how to process it. Without a CPU, we could not run programs on a computer.
At the hardware level, a CPU is an Integrated Circuit also known as a Chip. An integrated circuit "integrates" millions or billions of tiny electrical parts, arranging them into circuits and fitting them all into a compact box.Some of those layers are physical devices, like the chip and transistors, and some of those layers are abstractions, like logic circuits and gates.
It's impressive that we can put together seemingly simple devices like logic gates to create CPUs that power complex devices like our phones, computers, and even self-driving cars.