What is meant by the tribulation of the grave, and why did the Prophet (peace be upon him) seek refuge from it?

The trial of the grave involves the questioning of the deceased by the two angels. It has been established that the deceased will be questioned in their grave about their Lord, their religion, and their Prophet. As for the Prophet's—peace and blessings be upon him—seeking refuge from the trial of the grave, it is out of gratitude for blessings.