Hardware isn’t much use without software, the instructions that tell the brain what to do when you tap, click, type or talk.
The foundation for all software is the operating system. Windows, IOS, and Android are all operating systems. (There are more, but these are the most common.) The operating system takes care of organizing the details of how information gets from the input (keyboard, mouse) to the programs and from the programs to the output (printer, screen).
Programs make the hardware and operating system useful. Word processors, spreadsheets, web browsers, video and music players and recorders are all programs. An app is just another name for a program, often a more compact version used on phones.