不鸣则已，一鸣惊人

不鸣则已，一鸣惊人

Literally means:

when a bird need not cry, it doesn't, once it does, it will startle everyone

Actually means:

则, conjunction, indicating that two parts are conted. 已, as it is.

This phrase means that something or somebody remains unknown when nothing; it can be heard from them, but they can surprise people when they make an appearance. Metaphorically it means that a seemingly ordinary person achieves great success all of a sudden. This expression is used to compliment one's unexpected success. Commendatory.

则，连词，表示两件事的前后连接。已，罢了。

不鸣则已，一鸣惊人，要么不叫，一叫使人惊奇。比喻平时表现平平淡淡，忽然一下子做出了非凡的成绩。也比喻平时不显露才能，一旦显露出来，就有惊人的成绩。常用来称赞某种意想不到的成功。具有褒义色彩。