Foreshadowing is an advance sign or warning of what is to come in the future. The author of a mystery novel might use foreshadowing in an early chapter of her book to give readers an inkling of an impending murder.
When you want to let people know about an event that is yet to occur, you can use foreshadowing. Foreshadowing is used as a literary device to tease readers about plot turns that will occur later in the story. A fortune teller might use foreshadowing, warning that a short life line is a sign of some impending disaster.