To say that one thing equals another, as Romeo does here:
"worms that are thy chambermaids"
is to create a metaphor, and even specifically, anthropomorphizing the worms (making worms like humans).
Metaphors work like equations. Two unlike objects, people, places, etc., are compared directly, using the equation rather than the obvious comparison of a simile (where "like" or "as" is used).
What does it mean to say worms are chambermaids? This is an important question because figurative language creates a bigger image than saying something with adjectives or using generalizations, which aren't as vivid, compelling, or intriguing.
Several connotations or resonances are generated.
- Chambermaids extends the conceit (an expanded metaphor) of Death coming to Juliet like a bridegroom or lover in her bedroom. (Look earlier in the passage for Death being "amorous.") A chambermaid is a servant woman who attends a wealthy young woman. To say that chambermaids are present is to say that Juliet is cared for in her room. But ironically, how do worms "care for" the human body? What do worms do once we are dead?
- Romeo saying he will remain with these worms that are like chambermaids is making his decision to die easier. Worms (maggots) are disgusting in the sense of eating human flesh, so if Romeo "sweetens up" the image a bit, it might be easier for him to accept what he's about to do: suicide.
Hope this helps!