Foreshadowing is a literary device used to hint at later events in the story and often used to create suspense. It is achieved through clues and suggestions. Foreshadowing can be quite subtle and often only fully appreciated through a second reading of the work. It is important to note that foreshadowing occurs when there has been a significant interval of time between the clue and the event it foreshadows. 

Emily Dickinson offers a clever definition and illustration of foreshadowing in the following four-line poem:

Presentiment is that long shadow on the lawn
Indicative that suns go down;
The notice to the startled grass
That darkness is about to pass.

An example of foreshadowing can be found in this excerpt from Romeo and Juliet, where Romeo’s feelings of trepidation foreshadow his eventual demise:

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 despisèd life closed 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 (act 1, scene 4).

By contrast, the Prologue that opens Romeo and Juliet does not, strictly speaking, constitute foreshadowing, because it tells audiences explicitly what will happen at the end of the play.