Kun in Chinese / Japanese...

A 鲲 (Kun) is a famous sea monster, often associated or used interchangeably with a 鵬 (Peng).

鯤 is comparable to Leviathan or Jonah's whale.

In Chinese mythology, the Kun is a giant fish said to be able to turn into a bird.

鯤 and the mythological creature is also known in Japanese and Korean. However, in some context, this character can refer to fry (fish babies).

鵬程萬里 is an ancient Chinese proverb used in modern times to wish someone a long and successful career.

It's really about the 10,000 Flight of the Peng (Peng, also known as Roc is a mythical fish that can turn into a bird and take flight).

莊子
Zhuangzi or Chuang Tzu

Breaking down each character:
1. Peng or Roc (a kind of bird).
2. Journey (in this case, a flight).
3. 10,000 (Ten Thousand).
4. Li is a unit of distance often referred to as a "Chinese Mile", though the real distance is about half a kilometer.

Direct Translation: "Peng's Journey [of] 10,000 Li".
Literal meaning: "The 10,000-Li Flying Range Of The Roc".
Perceived meaning: "To have a bright future" or "To go far".

This proverb/idiom comes from the book of Zhuangzi or Chuang Tzu. It tells the tale of a huge fish which could turn into a gigantic bird. This bird was called "peng" and was many miles long. This legendary size allowed the Peng to fly from the Northern Sea to the Southern Sea in a single bound.

Wishing someone "a Peng's Journey of 10,000 Li", will imply that they will be able to travel far without stopping, and will have great success, a long career, and a prosperous future.