In this scene of Romeo and Juliet, by William Shakespeare, Romeo and his new wife, Juliet, have just passed their first night together as husband and wife. Romeo, having killed Tybalt in response to Mercutio's murder at Tybalt's hand, has been banished from Verona. If he is caught within the city at the break of day, he will be killed.
Romeo has especially suffered from this punishment: separation from Juliet is worse, in his eyes, than death.
Ha! Banishment! Be merciful, say 'death.' / For exile hath more terror in his look, / Much more than death. Do not say 'banishment.' (III, iii, 13-15)
"More light and light—more dark and dark our woes" paraphrased means as the sun rises (more light), our sorrows are much greater (darkened). However, the meaning with regard to the young couple is that with every moment that draws them closer to dawn, Romeo's banishment must be fulfilled or he will die. In other words, as the day begins, their sorrows increase because he faces the threat of death and they will be separated.