Juliet is describing her family tomb, because she is going to rest there while she pretends to be dead.
Juliet has decided to fake her death after Romeo is banished for killing her cousin Tybalt, but when she thinks about being in the family tomb she gets nervous. She thinks about the vault, the “ancient receptacle” and the fact that it is full of her family’s bones is not reassuring to her. She feels like she is being laid to rest with ghosts.
Juliet has many fears and questions as she prepares to take the poison. Will it work? Will it kill her? Will she wake up before she is woken up? Will she suffocate if she does?
This frightens her. She thinks about being alone in the tomb.
O, if I wake, shall I not be distraught,
Environed with all these hideous fears,
And madly play with my forefathers’ joints,
And pluck the mangled Tybalt from his shroud,
And, in this rage, with some great kinsman's bone(55)
As with a club dash out my desp'rate brains? (Act 4, Scene 4)
Juliet is worried that her family will not accept her, because she has betrayed them, especially Tybalt, by marrying Romeo. Romeo is her family’s mortal enemy, or at least his family is, and Romeo was the one who killed Tybalt. Juliet is a traitor to her family, yet she will be laid to rest with them. Despite the discomfort, Juliet is brave enough to take the potion.