An operating system is software that manages the resources of a computer - memory, central processing units (CPUs), etc. Both the operating system itself and the computer's users will need to execute programs. A program in execution is called a ``process', and every process needs a CPU (also called a processor) to run in. Very often, all of the CPUs will be busy, and so some processes will be waiting in a queue for their turn in a CPU.
How should the one or more CPUs that a computer has be shared among the processes? Perhaps the simplest strategy is first-come first-served, but it permits processes that need a great deal of CPU time to monopolize this essential resource. This problem can be alleviated by limiting the amount of CPU time that any process can have on one turn; if it needs more time, it must go back into the queue and wait for another slice of CPU time. Another consideration is process priority - some processes may be of higher priority than others. These are only a few of many issues that arise when deciding on a strategy for scheduling CPU time. Operating system designers often run simulations to help them decide what strategy to use.