When Juliet discovers Romeo below her window she fears that he has heard her secret confessions and wonders how he has successfully climbed the wall to her property. She is also probably very fearful that Romeo will be found on their property in what looks like a compromising position. In the Enotes analysis we see that "Juliet is startled and surprised that he has heard her secret thoughts. She asks how he was able to get over the high orchard walls and find her. To this, Romeo answers that love helped him accomplish both. Juliet is concerned that she has been too forward with him. She promises that she will be more true than any girl who acts shy and distant. Romeo tries to swear on the moon that he loves her; however, Juliet begs him not to swear on something that changes so frequently. The two lovers exchange vows of love, and Juliet asks if his intentions are honorable." Juliet is afraid that she has been too aggressive with Romeo, and she is fearful that her secret thoughts have been found out. Later in the Act the nurse calls for Juliet and Juliet is afraid Romeo will be found out.
I think this bit is great. You expect some sort of huge romantic moment when Romeo and Juliet finally meet at the balcony. Actually, Juliet is terrified:
What man art thou that, thus bescreen'd in night,
So stumblest on my counsel?
She's terrified. She doesn't know who this man is, as he's covered by the darkness of night. She does know that he's stumbled on her admissions about being in love with Romeo (her "counsel" - her information). But then, when she realises it is Romeo, there is another fear altogether:
How cam'st thou hither, tell me, and wherefore?
The orchard walls are high and hard to climb,
And the place death, considering who thou art,
If any of my kinsmen find thee here.
She is terrified that Capulet's men will kill him if they discover him on their property. The Capulets and Montagues, remember, hold an "ancient grudge" against each other.
Hope it helps!
Juliet is simply afraid that if someone were to see Romeo, such as the Nurse, considering the Nurse calls to Juliet during the scene, the penalty for Romeo would be death.