Review cycles allow you to group your reviews in batches. A review cycle has a start date and an end date. Those dates specify the period of time you want to use for the review, so when you ask someone to provide feedback on a coworker, he or she should only focus on behaviour observed during that period.
You can launch one survey and one appraisal per employee per cycle. If you want to start another round of appraisals or 360s, you probably need to define a new review cycle first.
Review cycles are also useful when comparing results with prior results for the same survey. For more information about survey reports please refer to Understanding 360° feedback survey reports.