In Act I, Scene 5 of Hamlet, the ghost, when identifying itself to Hamlet as the spirit of his dead father, gives the following explanation for its inability to go into more detail about its situation in the afterlife:
But that I am forbid
To tell the secrets of my prison-house,
I could a tale unfold whose lightest word
Would harrow up thy soul, freeze thy young blood,
Make thy two eyes, like stars, start from their spheres,
But this eternal blazon must not be
To ears of flesh and blood.
According to the Ptolemaic system of astronomy, the orthodox explanation for planetary and stellar movement during Shakespeare's time, the stars were fixed immovably to the surface of a gigantic sphere separated from the earth by a vast distance. If they fell to earth as comets or meteorites, it was the sign of an ominous shock to the natural system, often due to some horrible, unnatural, or immoral action in the world of men. For instance, Calpurnia, Caesar's wife, says of the celestial display before Caesar's assassination,
When beggars die there are no comets seen,
The heavens themselves blaze forth the death of princes. (Julius Caesar, Act II Scene 2)
Thus, what the ghost means by this simile is that the news of its true condition in the afterlife would be as terrifying a shock to Hamlet as some great disaster would be to the system of nature. It would throw him into total disorder, a disorder he would not be able to bear.