Valar Morghulis means "All Men Must Die." It is High Valyrian. Arya Stark is told these words and given an Iron coin by Jaqen H'ghar. We've seen her enter the House of Black and White with these words. There is so far no connection in the show between Jaqen and Daario except for the fact that they were both given coins, which may mean that there is a connection in who the coins come from and why.POSSIBLE SPOILERWe know also that Jaqen may or may not be the Many Faced God or just one of an order of assassins.** Daario was the best fighter in the pits, he could very well be an assassin, but we have so far seen no evidence that he was hired to carry out an assassination in particular at all.
Valar Dohaeris means "All Men Must Serve" It is customary to respond this way to Valar Morghulis.
Daario Naharis is a Tyroshi sellsword who in S3 is a lieutenant of the Second Sons. He is introduced played by a different actor, who comes to a meeting with the Second Sons and Daenerys Targaryan. He then returns played by Michiel Huisman, from S4 on. His role gets much larger as he takes over the Second Sons by killing the two leaders above him. He falls in love with Daenerys at first sight and eventually becomes her second/third in command, counseling her on many things, including re-opening the fighting pits. She falls in love with Daario, although she marries Hizdar for peace, which makes Daario angry. At the end of Season 5, Daenerys has flown off on Drogon, ending up surrounded by Dothraki. Daario and Jorah Mormont go off together to save her, while Tyrion runs the city of Meereen with Grey Worm and Missandei watching over the Unsullied. There are many theories of the true identities of both men. The question has not yet been answered. POSSIBLE SPOILER Daario could be the leader of the Sons of the Harpy, in some fan's opinions.**