an operating system
An operating system is the master program that runs your computer the moment you press the power button until you shut it down. Think of a busy hotel: the guests are the apps you want to run (a browser, a game, a music player), the building and its rooms are the hardware (the CPU, the memory, the disk, the screen), and the operating system is the entire hotel staff — the receptionist who decides who gets which room, the concierge who fetches things so guests do not have to deal with the plumbing themselves, and the security guard who keeps one guest out of another's room. No single app could keep all of that orderly on its own; the OS does it for everyone.
Concretely, the OS sits between your programs and the bare machine and does three big jobs at once. First it is a resource manager: it shares the one CPU, the limited memory, and the few devices among many programs so each gets a fair turn and none can hog everything. Second it is an abstraction layer: instead of forcing every app to know the exact electrical commands of your particular disk, it offers clean, uniform interfaces like open a file or send these bytes over the network. Third it is a control program: it watches over running programs, stops a faulty one from crashing the whole machine, and enforces who is allowed to do what.
Almost every computing device has one, even when you do not see it — your phone (Android, iOS), your laptop (Windows, macOS, Linux), the web servers behind a website, a car's entertainment system, and the tiny chip in a microwave all run an OS of some size. The OS is not the same as the apps you use, and not the same as the hardware; it is the layer that makes the hardware usable and the apps possible. Everything else in this subject — processes, memory, files, scheduling — is really a closer look at one of the jobs the OS is quietly doing.
You double-click a music app while a download runs and a chat window is open. Three programs want the CPU, all want memory, and two want the network — yet they coexist, and clicking close on one does not freeze the others. That smooth coexistence is the operating system at work, slicing the CPU, parcelling out memory, and refereeing the devices behind the scenes.
The OS makes one shared machine behave like a private one for each program.
Common confusion: the OS is not the desktop wallpaper, the Start menu, or the app store — those are programs that come bundled with it. The real OS is mostly the kernel and a set of system services you never directly see.