A figure of speech is often called an idiom. This is a special phrase that has entered into common usage, in this case as a result of its famous use by Shakespeare. Shakespeare used them to make his writing more interesting, and to bring up images in the reader or audience member’s mind. Today, we use these phrases for the same purposes, and so that everyone will know what we are talking about, because they have entered our common culture.
In the prologue, Romeo and Juliet are referred to as “a pair of star-cross'd lovers” (enotes etext PDF p. 8). This means that the lovers were fated from the stars, or meant to be. It has entered common usage since then.
In Act II, Scene 2, Juliet tells Romeo that his name does not matter, producing one of the most famous Shakespearean figures of speech.
What's in a name? That which we call a rose
By any other name would smell as sweet. (p. 39)
Today it is more commonly said as “a rose by any other name would smell as sweet,” leaving off the first few words.
In Act V, Scene 3, Juliet kills herself with this line. It is ironic because she is happy that the dagger is going to kill her, and describes the dagger as happy.
Yea, noise? Then I'll be brief. O happy dagger! (p. 111)
Today, “O’ happy dagger” is usually used ironically to refer to something that is not good.
In Act II, Scene 4 there is a phrase you all must know.
Nay, if our wits run the wild-goose chase, I am done (p. 47)
We still refer to a wild-goose chase as looking for something that is impossible to find.