Each process gets a CPU time quantum, usually 10-100 milliseconds. After that, it is preempted and added to the end of the ready queue.
If there are n processes in the ready queue and the time quantum is q then each process gets 1/n of the CPU time in chunks of at most q units at once. No process waits more that (n-1)q time units to be scheduled.
- Large q => FCFS.
- Small q => Context switch overhead!