At the beginning of this scene, Romeo is with Juliet in her garden, and it is daybreak.  Romeo is readying to leave, and Juliet is trying vainly to prevent him from going, asserting that the lark he hears is in fact a nightingale; a herald of darkness, rather than light.  When she states that “yon light is not day-light,” but instead “some meteor that the sun exhales,” she is further attempting to convince Romeo to stay; to convince them both that morning has not come to end their tryst.  Romeo, somewhat jokingly, replies, “Come, death, and welcome!  Juliet wills it so./How is’t, my soul?  let’s talk; it is not day.”  To which Juliet replies that he must leave, having realized how selfish and short-sighted her words were.

These images serve to emphasize how ardent the two young lovers are, and how deeply they feel about each other – that by merely saying that the day is not come, one could convince the other that it is so.  That one would readily deny what he sees with his own eyes, simply because the other says that it is not the truth.

In addition, later in this scene Juliet learns of her arranged marriage with Paris, to take place only a few days later.  So, much as the day comes, unstoppable, to end her time with Romeo, so does her love seem to be coming to a permanent end with the news of this union.  And just as Juliet tries to deny the obvious signs that the sun is rising, she attempts to deny her father and deny Paris – yet one cannot prevent the sun from rising, and one cannot prevent one’s fate.

In this part of the scene, Juliet and Romeo are trying to convince each other that the sun is not coming up and Romeo does not need to leave.

I think that the most important aspect of the imagery that Juliet uses in the lines you cite is the image of a meteor.  I think that her use of that image is something of a metaphor for their love and even their lives.

If you think about it, their love really is a meteor.  It burns very brightly, but only for a very short time before it is gone.  So I think this is sort of foreshadowing how brief their love will be.