祥瑞 is a concept deeply embedded in Chinese history and philosophy, particularly connected to the idea of 天命 (tiānmìng), the “Mandate of Heaven.” According to this belief, a just and virtuous emperor would be blessed by Heaven, and this blessing would manifest as 祥瑞 appearing in the kingdom. The appearance of a mythical creature like a 麒麟 (qílín - a Chinese unicorn) or a 凤凰 (fènghuáng - a phoenix), or even a natural event like a timely, nourishing snow (called a 瑞雪, ruìxuě), was considered concrete proof of the ruler's legitimacy and a sign of impending peace and prosperity. A useful Western comparison is the concept of an “omen” or “portent.” However, there's a key difference. In Western culture, an omen can be either good or bad (e.g., a black cat crossing your path is a bad omen). 祥瑞, in contrast, is exclusively positive. It is always a sign of good things to come. It embodies the traditional Chinese value of 天人合一 (tiān rén hé yī), or the harmony between humanity and the cosmos, where a virtuous society is rewarded with signs of cosmic approval.
祥瑞 is a formal, somewhat literary term. You are unlikely to hear it in everyday, casual conversation about getting a good grade or finding a twenty-dollar bill. However, it is still very much present in modern China.