Theimagery that Shakespeare uses paints Queen Mab to drive people mad with desire; instead of ordinary “romantic love,” the love that Queen Mab would push someone to is more like lust or obsession—the same urging that pushes lawyers to dream of fees or soldiers to dream of slitting throats pushes lovers to dream of love.

The reason Shakespeare uses this mad type of love is explained when Friar Lawrence when he says,

These violent delights have violent ends
And in their triumph die, like fire and powder,
Which, as they kiss, consume. (act 2, scene 6)

The love that Shakespeare describes in Romeo and Juliet is a dangerous, fanatical love. It is like powder primed and ready to explode, which is the reason that the two lovers are pushed to suicide, the most dramatic and unchangeable decision. If they cannot be together, they would rather be dead—that sounds exactly like what Mercutio describes when talking about Queen Mab.

In Act I, Scene 4 of Romeo and Juliet, Mercutio tells Romeo that Queen Mab has visited Romeo in his sleep. Queen Mab is a curious creature who is "no bigger than an agate stone/ On the forefinger of an alderman" (I.4.56-57). In other words, she is a tiny creature who is as big as a ring. Her mini wagon is pulled by a gnat. She rides through lovers' brains and makes them dream of love. When Queen Mab drives over the lips of women, they, "straight on kisses dream, /Which oft the angry Mab with blisters plagues, /Because their breaths with sweetmeats tainted are" (I.4.77-79). In other words, Queen Mab rides over women's lips, and they dream of kisses. Then, Queen Mab gives them blisters because their breath smells like candy, which she does not like. This story is a metaphor for the idea that love can bring with it suffering and punishment. Shakespeare used this story to foreshadow the punishment that will come to the lovers, Romeo and Juliet.

If you think of metaphor as comparing two unlike things where one equals or is the other without using the words like or as, and then hold one unlike thing in each hand to make sure you really do have two unlike things to compare, metaphor will be easier. For example, in the Queen Mab speech, Mercutio says that, "her chariot is an empty hazelnut" which is a metaphor comparing her chariot to an empty hazelnut.  You have two objects which are unlike each other saying that one IS the other rather than like each other which is what a simile does.  Another example in the same speech is a bit more subtle because the word "is" is not used but implied.  "Her whip of cricket bone" or even "her wagoner a small gray-coated gnat" are both implied metaphors comparing two unlike objects,the whip with the cricket bone or the wagoner with the gnat, without using the words like or as.  Many more metaphors are in this speech which you can find if you remember the definition and find the two objects to hold in each hand as you compare. Obviously, holding the objects in your hand is not a literal direction, but it will help you figure out metaphors. 

This is of course one of the most famous speeches in the entire play. Queen Mab is the mythological figure who brings dreams to people while they sleep. However, critics have focused on the way that her name actually has a less savoury connotation. The words "quean" and "mab" were actually used to refer to prostitutes in Elizabethan times. This speech therefore represents something of a pun as a childish myth is linked to a rather bleak vision of humanity and its darkness. This is paralleled by the way in which the description of the dreams Queen Mab brings to each person is attractive in a fairy-tale kind of way, but also becomes increasingly darker, until soldiers are said to dream of "cutting foreign throats." At the end of the speech, when Mercutio is losing control of his emotions, Queen Mab is the "hag" that initiates girls into sexual knowledge:

This is the hag, when maids lie on their backs,
That presses them and learns them first to bear,
Making them women of good carriage.

What began as a simple child's tale of fairys has now devolved into something much more sinister and disturbing, which we could also argue represents a much more realistic vision of society. Mercutio seems to be offering an alternate view of the grand tragedy in this play, and Mercutio ends by saying that dreams "are the children of an idle brain." Mercutio's comment about dreams can be seen as giving a harsh verdict on not only Romeo's dreams of love, but the Friar's dreams of peace and the various other dreams that characters have. All, according to Mercutio, are nothing more than delusions. The Queen Mab speech can therefore be seen as offering an alternate view of the play that goes some way towards deflating the grand action and reminding us of a darker view of humanity.