At this point, Juliet is utterly distraught. She had just recently married Romeo in secret, had discovered that her love had killed her much-loved cousin, Tybalt, was told that Romeo had been banished on pain of execution, and finally, had wished him an excruciatingly sad and poignant farewell.
Obviously, her parents' forcible insistence that she marry Paris on Thursday could not have come at a worse time. Juliet seeks more occasion so that she may, at some stage or another, join Romeo. If she were to marry Paris that Thursday, she would not have an opportunity to do so.
Lord Capulet is angry with Juliet for being so reticent to marry Paris and has pledged to disavow and disinherit her if she does not obey his command. It is then that Juliet lodges her desperate plea:
"Delay this marriage for a month, a week;
Or, if you do not, make the bridal bed
In that dim monument where Tybalt lies."
Juliet stresses that if her parents do not agree to her request, they might as well bury her with Tybalt, where of course, death would be her bridegroom and the dark Capulet crypt would be her bridal bed. Juliet's words foreshadow what is to happen later - when she would, in fact, become death's bride and be entombed in the family vault after she commits suicide.
In this scene, Juliet's mother and father have been talking to her, telling her that she is going to marry Paris and she is going to do it right away. She has said she doesn't want to and her father has gotten really angry at her. Her mother is not pleased either.
What Juliet tells her mother (her father has already stormed out of the room) is that she might as well just kill Juliet if she won't delay the marriage. She says to just make her bridal bed be in the tomb next to Tybalt.
In the play, Juliet says that if her mother doesn't delay the mariage for #amount of time, she might as well make her wedding bed in the tomb where Tybalt lays.