Romeo speaks this line in act III, scene v of Romeo and Juliet. This scene comes at the end of the night which the two young lovers have spent together. Romeo thinks he should leave Juliet’s house while it is still dark out so nobody sees him. He has been sentenced to banishment for killing Tybalt and is trying to avoid being captured before he can escape to Mantua.

In act II, scene ii, Romeo had stood alone in the dark below Juliet’s balcony. As he saw her lovely face at her window, he addressed the sun and observed that the moon was jealous of Juliet because she is more beautiful. His current comment about the “envious streaks” builds on that earlier claim.

As this scene begins, the two of them are looking out Juliet’s bedroom window. Knowing that he should leave before it gets light, she is trying to pretend that it is still night. They are having a small disagreement about whether it is light or dark outside. As they hear a bird singing, Romeo says it is a lark, which sings in the daytime. She counters by saying it is a nightingale, which sings at night. Romeo insists that it is a lark, and to support his claim, he points to the east where the sun is rising:

It was the lark, the herald of the morn;
No nightingale. Look, love, what envious streaks
Do lace the severing clouds in yonder East.

The “streaks” are the first bits of sunlight peeking up over the horizon. Still in denial, Juliet then says it must be a meteor. As Romeo reminds her that if he is caught, he will be killed, but it would be worth it to stay with her. This talk of death changes her mind, and she admits it was a lark and urges him to leave.