"Reow" is a phonetic word from the flash animation "End of the World" where many major world powers nuke each other.
It is used is the following ways:
1. A Greeting - like Hello
2. A Congradulatory comment - like Good Job! or Cool!
* Other comments from the animation may also be used in conjunction, such as "H'yes" or "h'okay".
** "Good Good" a reference to a Family Guy episode, may also be used to humor the dialect.
Brian: Hello Sean
Brian: I just got a PSP!
Brian: H'yes, good good.