When you train in martial arts, one of the first things you learn is not to telegraph your punches. The minute you start showing your opponent what you're going to do next, the fight is over. 

Telegraphing in novels is almost as bad, because it ruins the tension and keeps the reader from caring about what happens next (because you've just told them, rather than showing him through the narrative). Be careful with sentences like "he had no idea that" or "unbeknownst to her" or anything similar. These are typical beginnings for sentences that telegraph what is going to happen in the next scene, the next chapter or (worse) the rest of the book. 

A lot of novice writers use sentences like this that act like a wind-up punch the reader's going to see a mile away. It comes across as overly dramatic (and often might have an unwritten, "dun dun duuuuun" after it) and indicates an inexperienced writer as well. 

Learn to look for instances of telegraphing like this in your own work. When you find them, ask yourself "what is the purpose of this phrase?" Often, it is to instill a false sense of tension and (in the writer's mind) set up conflict to come. 

This is, of course, lazy writing. If anything is told, or telegraphed by telling, then it is not being shown to the reader. And as everyone has heard, showing is superior to telling in ninety-nine percent of the time. This is not just because I said so, but also because by showing the story to the reader, you are allowing them to engage with the story on a level that you don't get by merely telling them what happens. 

If your goal is to engage the reader and get them to feel like they're a part of the story. You aren't just letting them watch a movie in their mind, you're letting them live it through your words. By telling them any part of your story except for the most non-vital information, you are depriving your readers of an way to experience the story and it will be less engaging, and, ultimately, less interesting. 

Therefore to engage the reader, you need to figure out how to work tension into your work without using telegraphing phrases. This involves many factors (and can be the topic of an entire book), but tends to rely on plot, characterization, setting, and even theme. It may be hard to develop tension, however, it only takes a phrase to destroy it.