What it means
Goodbye. Literally "again see" — see you again.
When to use it
For any departure, formal or casual. Friends often just say 拜拜 (bāibāi), borrowed from English.
Examples
wǒ我xiān先zǒu走le了，zài再jiàn见！
I'm heading off, goodbye!
zài再jiàn见，míng明tiān天jiàn见。
Goodbye, see you tomorrow.
zài再jiàn见，lù路shàng上xiǎo小xīn心。
Goodbye, take care on the way.
The full set of survival phrases for greetings, dining, shopping, and getting around, each with audio.