Juliet and Romeo have spent the night together as man and wife before he heads away to Mantua, to where he has been banished.  The Friar has assured them both that this banishment will not last long, and they will soon be together again.  As Romeo is leaving, Juliet first says this:

Art thou gone so, my lord, my love, my friend?
I must hear from thee every day in the hour,
For in a minute there are many days.

She is asking to hear from him as much as possible while he is gone.  She reveals in the next line that she is concerned that she will be much older when they finally get to see each other again:

O, by this count I shall be much in years
Ere I again behold my Romeo!

As she continues speaking, she reveals that she is concerned that this is their last meeting:

O, think'st thou we shall ever meet again?

Shakespeare is using Juliet's natural fears as a way of reminding us, the audience, of what he told us in the prologue.. that these "star-cross'd lovers" are doomed to die.  One of Juliet's last lines to Romeo in this scene underscores that idea:

Methinks I see thee, now thou art below,
As one dead in the bottom of a tomb.

The next time she sees him, he will be dead at the bottom of a tomb.  This is an example of foreshadowing, because it tells us what is to come, but because Shakespeare already told us this in the prologue, it is also an example of irony.  We know something that the characters don't, so their words have more meaning for us than they do for them.