The dead bodies of Romeo and Juliet lie together in the Capulet family tomb. As one can imagine, everyone is shocked and saddened by the tragic demise of these two young lovers, cruelly snatched from the world much too soon.
Lady Capulet, Juliet's mother, is especially hard-hit by the terrible sight of Romeo's and Juliet's dead bodies. As soon as she casts eyes upon them, she is plunged into unimaginable grief:
O me! This sight of death is as a bell,
That warns my old age to a sepulcher.
(act 5, scene 3, lines 221–222)
In other words, the death of Romeo and Juliet is like the ominous tolling of a bell that reminds Lady Capulet that she is old and will soon die herself.
In Shakespeare's day, and for many years after, it was quite common for a bell to be tolled when someone was about to die, when they actually died, and when they were buried. The tragic death of Lady Capulet's daughter and her beloved Romeo seems to her like a passing bell, which was rung to warn someone of impending death.
Though Lady Capulet exaggerates somewhat, allowances must be made for her grief-stricken state of mind. Under the circumstances, it's perfectly understandable that she should feel that her life is almost at an end. Lady Capulet is so utterly devastated by these heartbreaking deaths that she's convinced that she will soon be joining Romeo and Juliet in death.