Shakespeare uses Mercutios" fantastical Queen Mab speech to foreshadow future events in the play. Romeo is very affected by the speech because it is he who has strong desires.
Mercutios speech is about Queen Mab who is the mid-wife or the creator of desires. Queen Mab comes in your dreams and gives you your desires. When you desire too much she makes you suffer and pay the consequences. Romeo has strong desires for a storybook love and for a girl named Roselyn. Romeo feels that he will have to pay the consequence of death for his desires: L115 " I fear too early for my mind misgives some consequence yet hanging in the stars shall bitterly begin his fearful date with the nights revels, and expire the term of despised life closed in my breast by vile forfeit of untimely death but he that hath the steerage of my course direct my sail. On lusty gentlemen." When Romeo said this he means that in his future he will have to pay for the desires he has and his consequence will be an early death.
Mercutios speech had convinced Romeo that desire caused pain. And since he desired so strongly he feared that he would have to pay for the desires. .
Romeo feels that somewhere in his future his consequence will come Romeo states this in L 114 "some consequence yet hanging in the stars shall bitterly begin his fearful date." Here Romeo foreshadows what is to become of him by the end of the play. Romeo does indeed have an untimely death in the end and suffers the consequences of those desires by the death of his true love Juliet and his own suicide.