First let's explain what a process is. A process is a program that is being run on your computer. Today computers allow for multiple programs to run at the same time. These programs can be anything from a Microsoft Word document to your anti-virus software to your background windows programs running on your computer... all at the same time!
So how does your computer manage all these programs (or processes how we will refer to them from now on in this article)? This is done through something called multi-programming.
The main objective of multi-programming is to keep on running programs all the time for maximum CPU utilization. Scheduling these processes is the fundamental function of the OS. The task of selecting the processes in memory that are ready to execute, and allocating them to the CPU is performed by the CPU Scheduler. Once this is done, the OS rotates the processes.