“And he shall put the incense on the fire before Yahweh, that the cloud of the incense may cover the mercy-seat that is on the testimony, that he die not,”
And there he must burn the incense on the coals of fire so that the cloud from the incense covers the mercy seat that is over the tables of the Law, hiding it from his gaze. The implication is that otherwise he would die. The censer is then left in the Holy of Holies so as to continue producing the cloud.