In Act Three, Scene 5, Lady Capulet tells Juliet that she will marry Paris on Thursday. Juliet is shocked and disturbed at this news. Juliet then tells her mother that she refuses to marry Paris so soon. When Lord Capulet enters the scene, he asks his wife where Juliet stands on their decision to have her marry Paris. Lady Capulet responds by telling her husband, "Ay, sir, but she will none, she gives you thanks. I would the fool were married to her grave!" (Shakespeare, 3.5.139-140). Lady Capulet is essentially calling her daughter a fool and wishes that Juliet were dead for not obeying their decision. Lady Capulet is extremely disheartened that Juliet refuses to marry Paris. Her comments are also significant because they foreshadow Juliet's death later on in the play. Lord Capulet is initially confused after hearing his wife's comment, and Juliet elaborates on her decision not to marry Paris. Lord Capulet responds by ridiculing his daughter and mentions that Juliet disgusts him.
Because Juliet will not agree to marry Paris, Lady Capulet states the above quote (line 140). Lady Capulet is stating that it would seem that Juliet is married to her grave because she would rather be with Romeo than marry Paris (Marrying Romeo would be the death of Juliet, figuratively, as she would be marrying into the Mercutio family). This line is also used as an element of foreshadowing, as Juliet does actually die at the end of the play.
It is also a part of the recurring theme that Juliet is married to death.
"If he be married, my grave is like to be my wedding bed."
"and death, not Romeo, take my maidenhead"
"Delay this marriage for a month, a week
Or, if you do not, make the bridal bed
In that dim monument where Tybalt lies"
"Hath Death lain with thy wife; there she lies,
Flower as she was, deflowered by him.
Death is my son-in-law, Death is my heir"
"Sweet flower, with flowers thy bridal bed I strew"