What is this rising from the dead?

τί ἐστι τὸ ἐκ νεκρῶν ἀναστῆναι[;]

     Mk 9:10:  "And as they were coming down the mountain, he charged them to tell no one what they had seen, until the Son of man should have risen from the dead.  So they kept the matter to themselves, questioning what the rising from the dead meant" (RSV).