The round-robin scheduling algorithm allocates CPU time to processes by sequentially assigning the CPU to processes of equal priority that are in the state of being able to use the CPU. (Not blocked)
This works by appearing to evenly distribute the CPU amongst CPU ready processes. Processes that are waiting on something, such as an I/O event, particularly waiting on the user to press Enter, are not considered for allocation.
Often, there is a priority assigned to the process, which factors in the allocation strategy. Processes that are mostly I/O intensive tend to have higher priority, giving them good response time. Processes that are mostly CPU intensive tend to have lower priority, so they don't interfere with overall system responsiveness.