Valar Morghulis is a common greeting in Braavos, meaning "all men must die" in High Valyrian. It is meant in the sense of "all men must (eventually) die," sooner or later, somewhat like the Latin saying "memento mori." The customary response is Valar Dohaeris— "all men must serve."
Valar dohaeris!
"Valar Dohaeris" is High Valyrian, the response phrase to "Valar Morghulis," the title of the Season 2 finale. Commonly used in Braavos, "valarmorgulis" means "all men must die"