Your laptop seems to run music, a browser, and a chat at once, but a single CPU is really switching between them thousands of times a second — too fast to notice. The operating system is that juggler creating the illusion of many programs running together.
The great illusion
An operating system sits between your programs and the hardware and creates useful illusions. A process is a running program with its own memory. Your laptop seems to run dozens at once, but a single CPU core runs just one at a time — the OS switches between them thousands of times a second, so fast it feels simultaneous.
To switch processes, the OS saves the exact state of one (its registers, its place in the code) and restores another's. The cost of saving and restoring is real overhead — switching too often wastes time.