A roc is a mythical giant bird. Though in this case it is the Chinese "peng" which apparently is somewhat different:Thank you!
"the fabled roc flies ten thousand miles" (idiom) / (one's future prospects are brilliant)
What is a "roc"?
To tell the truth, I didn't know this four characters idiom, and got the meaning by googling it.Do people actually use this expression 鵬程千里 in normal conversation?
(In modern Chinese it is a common enough expression that I knew it already despite my low level, and it can be something you say to wish someone else success, like "I hope you will go far in your life/career".)