Kylo says

> "The Supreme leader is dead"

to which Hux replies 

> "Long live the Supreme Leader".

Basically they're confirming that [the \[old\] Supreme leader (Snoke) is dead so they now need to transfer their wish of long life to the \[new\] Supreme Leader (Kylo Ren).][1]

  [1]: https://en.wikipedia.org/wiki/The_king_is_dead,_long_live_the_king!