How does Juliet "speak, yet ... [say] nothing"?
As Romeo stands beneath Juliet's balcony, he romanticizes the vision of Juliet, imagining that she transcends nature, elevating her in the Elizabethan chain of being:
...What light through yonder window breaks?
It is the east, and Juliet is the sun!
Arise fair sun, and kill the envious moon,
Who is already sick and pale with grief
That thou her maid art far more fair than she.....
Her eye discourses, I will answer it
I am too bold, 'tis not to me she speaks. (2.1.2-14)
Romeo's words are in the tradition of courtly love in which the man elevates the woman to a source of inspiration and reverence. Such poetry is designed to lead to a spiritual enrichment between the woman and the man who loves her.
Juliet's "eye" speaks as she contemplates the metaphysical world; she is held in a realm above Romeo, in the tradition of courtly love. When Romeo describes her eyes this way, he implies that Juliet is a superior being whom he must worship from below. In his humility, Romeo wishes that he might be a glove so that he could touch her fair cheek that Juliet leans upon her hand.
Act II, Scene 2 is magnificent poetry, splendid in its imagery and enchanting in its metaphor. Romeo praises his love; she is superior to the celestial beings, her thoughts are heavenly and she communicates with the ethereal world through her lovely eyes. Juliet's sighs excite him in his passionate love for her to the point that he would be thrilled if only he could touch her cheek.
Oh, that I were a glove upon that hand
That I might touch that cheek! (2.1.22-23)
This quotation comes from Act II, Scene 2 of William Shakespeare's Romeo and Juliet. The full line is as follows: "She speaks yet she says nothing: what of that?"
This quotation is spoken by Romeo, who is standing in the Capulet's orchard beneath Juliet's window. Romeo has just seen Juliet and is praising her beauty. Juliet does not yet see or hear Romeo.
I would imagine that the key to understanding Romeo's meaning here is the line that follows the "She speaks" line. Romeo continues to say "Her eye discourses; I will answer it."
As many people discover, sometimes people can speak without saying a word. Sometimes people who know each other very well, or think they know the other person well, can understand what the other person has in mind without the other person having to speak. Best friends, brothers, sisters, and people in love can often understand what the other person is saying even when they are not saying anything.
In Juliet's case, I imagine that Romeo thinks her eyes are so beautiful that they speak volumes.