What is the trial of the grave's torment? And does it refer to the torment of the grave or something else?

It is not said "fitnat 'adhaab al-qabr" (trial of the punishment of the grave), but rather "fitnat al-qabr" (trial of the grave) and "'adhaab al-qabr" (punishment of the grave). The trial of the grave is the questioning of the deceased by the two angels; the believer remains firm, while the hypocrite and the disbeliever do not remain firm and are punished. None are saved from it except the martyr and the one who guards the frontier (murabit). As for the punishment of the grave, it is affirmed by the Qur'an and Sunnah for those who deserve it.