According to the website www.foreshadowing.org, "to foreshadow means to indicate or suggest something that is going to happen later, either good or bad."
An example of foreshadowing in Of Mice and Men occurred when Lenny wasn't allowed to get a dog, because he wasn't gentle enough. At the end of the story he ends up killing Curley's wife.
In The Outsiders the author uses foreshadowing when Ponyboy says to Johnny, "if that old church ever caught fire, there'd be no stopping it". Later in the story the church burns down.
Foreshadowing is useful because it builds suspense in literature. It's not only useful in literature but in movies as well. The audience can try to guess what will happen later in the story.