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)  .

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).