An operating system is a master control program that manages the running of the computer system. In all environments, except for specialized scientific and process control applications, the operating system interacts with the application programs. The application programs "talk" to the operating system.
If application programs are moved to a different computing environment, they have to be converted to interface with a different operating system. If a new operating system is installed that is not compatible with the old one, the application programs have to be converted to the new operating system. Before personal computers, the lay person did not have a clue why there was so much conversion going on in the datacenter. However, the 1990s changed all that as users learned what it meant to upgrade applications from DOS to Windows 3.0; then from 16-bit Windows to 32-bit Windows 95 and so on. See operating system