Most writers will integrate some level of foreshadowing into their first draft on instinct. Where it really makes its impact though, is in the 2nd draft when you have time to take a good look at where you want to tease your reader, give hints about the future, and lay the groundwork for a big surprise yet to come.
At its simplest, foreshadowing is when the writer sets the tone/atmosphere at a particular moment in the story to add drama for an important event or reveals a nugget of information that seems innocuous at the time but has a greater impact on plot or character development later in the story.
You’re manipulating your reader into believing something big is about to happen (but what?) or that some trivial piece of information might actually be significant for some reason in the long run.
Employing this technique is not mandatory but adding suspense and tension to your story is – this is just one tool for helping you get the job done.
Here’s a great little resource to help you strengthen your understanding of and use of foreshadowing:
It’s a concise little article and a great resource blog to help you during you on your writing journey.