In the Southern and Northern Dynasties (nán běi cháo 南北朝,420-589 AD),Liang Wudi (liáng wǔ dì 梁武帝),the ruler of Liang Dynasty (liáng cháo 梁朝) had a strong belief in Buddhism,therefore,he exerted himself to propagandize this religon.

At that time,there was a monk called Master Yun Guang (yún guāng dà shī 云光法师) who was a very accomplished preacher. Once he explained the sutra so proformdly and subtly that the God of Flowers was moved and sent divine flowers down to earth. Soon the land was covered with flowers.

This idiom was later used metaphorically to describe talking in a vivid and eloquent way,mostly in an exaggerated and impartical manner.

南北朝时期，南朝的梁武帝带头求神拜佛，在全国大建寺庙。有一个名叫云光的法师，讲经讲得很好。传说他有一次登坛讲经，感动了天上的花神。花神把天上的花纷纷降落下来，洒遍了大地。

这个成语后来比喻说话有声有色，非常动听,多指夸大的或不切实际的事情。

天花乱坠 (tiān huā luàn zhuì)
【翻译】As If it were Raining Flowers.
【释义】传说梁武帝时有个和尚讲经，感动了上天，天上纷纷落下花来。形容说话有声有色，极其
            动听。现在多指夸张而不符合实际。
【例句】现在很多厂商都喜欢把自己的产品吹得天花乱坠，其实质量并不如他们说的那么好。
【近义词】不着边际、花言巧语
【反义词】平铺直叙、兴味索然