Here are Romeo's words that illustrate the metaphor to which you refer:
Thou detestable maw, thou womb of death,
Gorged with the dearest morsel of the earth,
Thus I enforce thy rotten jaws to open,
And, in despite, I'll cram thee with more food!
A maw is a mouth. Romeo furthers the metaphor by using the words gorged, morsel, jaws, and food. Each of these words not only has to do with the tomb being a mouth, but the door a vessel that leads to eternal consumption. This is what a mouth does. The morsel he refers to is Juliet, and the more food that he wants to cram it with is himself.
An additional metaphor is comparing the tomb to a womb. A womb is the place of a woman's body that is pregnant with new life, so I think he is trying to illustrate how much life Juliet was to him.
In Act V, Scene 3, Romeo uses a metaphor as he compares the tomb to the mouth of death:
Thou detestable maw, thou womb of death,Gorged with the dearest morsel of the earth,Thus I enforce thy rotten jaws to open,And in despite I’ll cram thee with more food! (5.3.51-54)
Having told Balthasar to leave, Romeo enters the Capulet tomb in order to find Juliet's resting place. He addresses the opening of the tomb first as a "detestable maw," a metaphor that compares the tomb to the jaws of a hungry beast. Then, he speaks of Juliet's tomb as "the womb of death," a metaphor that compares the burial place to a womb in which decay rather than growth takes place.
Both of these metaphors apply. Romeo intends to feed the "maw" first with the body of Paris, whom he has killed. Then he will feed it with his own body after dying beside Juliet.
However, for Juliet the tomb is not at first the "maw" of death; instead, it is like a womb, as she lies within it in an unconscious state. When the drug given to her by Friar Lawrence wears off, Juliet becomes like a baby that has been nurtured in the womb until it emerges in birth. Tragically, however, after her "rebirth" from this womb, Juliet's tomb again become a "maw of death" when she kills herself after realizing that Romeo has died from drinking poison in despair.