Romeo accompanies Benvolio and Mercutio to the Capulet party, but he is filled with misgivings. He tells Mercutio that it is "no wit"(not a bright idea) to go the party. When Mercutio asks why, Romeo replies he had a dream the night before. Mercutio says that dreams often lie. Romeo responds,
In bed asleep ... they [people] do dream things true.
Mercutio then tells of the false ideas Queen Mab puts into people's heads while they sleep, but he is unable to shake Romeo's sense of foreboding, which Romeo returns to in lines 106–113. Romeo insists that he has a bad feeling about the party, saying that
Some consequence yet hanging in the starsShall bitterly begin his fearful date.
Poor old Romeo's feeling rather down in the dumps, as he's not making much headway with Rosaline, for whom he's developed an infatuation. In order to cheer him up, his good friends Benvolio and Mercutio urge him to come along with them to the Capulets' party. They hope that Romeo will have a good time there and forget all about Rosaline.
Romeo, however, is far from enthusiastic. Not only is he not in the party mood, he also senses that going to the party will be the start of something bad. He doesn't know quite what that something is, but he feels pretty uneasy about it all the same, not least because he's convinced that it will end in his own tragic demise:
Some consequence yet hanging in the starsShall bitterly begin his fearful dateWith this night’s revels, and expire the termOf a despisèd life closed in my breastBy some vile forfeit of untimely death. (I, iv, 108–12).
Sadly, Romeo's absolutely right. At the Capulets' party, he first lays eyes on, and falls head over heels in love with, Juliet. In some respects, this is a good thing, as it brings Romeo untold happiness. It also has the very happy consequence of making him forget all about Rosaline.
But in the end, the love between Romeo and Juliet leads directly to their tragic, untimely deaths, so Romeo's premonition turned out, unfortunately, to be all too accurate.
Further Reading
This speech is essentially Romeo's "Something's Coming" moment. In this scene, Romeo, Mercutio, and Benvolio are all headed to crash the Capulet party. Though going to see Rosaline, Romeo appears to have given up hope of making Rosaline fall in love with him and is only joining his friends so that he can sulk:
I am too sore enpiercèd with his shaftTo soar with his light feathers, and so bound,I cannot bound a pitch above dull woe.Under love’s heavy burden do I sink. (1.4.19-21)
I fear too early, for my mind misgivesSome consequence yet hanging in the starsShall bitterly begin his fearful dateWith this night’s revels, and expire the termOf a despisèd life closed in my breastBy some vile forfeit of untimely death.But he that hath the steerage of my course,Direct my sail. On, lusty gentlemen.
For the avoidance of confusion, I will first paste act 1, Scene 4, lines 106-113 below, indeed a speech by Romeo:
I fear, too early, for my mind misgives
Some consequence yet hanging in the stars
Shall bitterly begin his fearful date
With this night’s revels, and expire the term
Of a despised life clos’d in my breast
By some vile forfeit of untimely death.
But He that hath the steerage of my course
Direct my sail! On, lusty gentlemen!
What Romeo is saying here is that he has a strange feeling that "some consequence yet hanging in the stars"—that is, something that is fated—will be set in motion "with this night's revels." He fears that whatever happens at the ball will only be the beginning of something which will, in the end, "expire the term / Of a despised life clos'd in my breast." Effectively, he fears that in going to the ball, he will somehow be sealing his own fate, which will end with his "untimely death." This premonition foreshadows what happens later on in the play, because, of course, Romeo is about to meet Juliet, and it is this meeting which will set in motion the tragic events of the rest of the play.
However, despite this strange feeling, Romeo still goes to the ball, trusting that fate cannot be altered, but is in the hands of God: "He that hath the steerage of my course." God is the one who has determined the path Romeo will travel, so Romeo cannot do anything to alter it.
This speech can be compared to Romeo's final speech at the end of the play, when, having laid Paris in the grave with Juliet, he determines to "shake the yoke of inauspicious stars"—defy fate, or at least claim some agency in his own destiny—by killing himself, thus determining the time of the "untimely death" he somehow knew was coming.