Tension (and/or suspense) is a key element in storytelling. It is what keeps a reader turning pages past their bedtime. According to Gary Provost, tension is "a cord or series of cords that stretch across every paragraph you write".
Tension is the "anticipation and dread" factor in your story. Your readers look forward to the consequences of your character's action and the uncertainty surrounding those consequences. Tension creates dread. Will the young soldier die trying to save his comrade? Will the Lady lose her reputation and be forced into a convent? How will an unemployed single mother take care of her children? Without uncertainty present, without tension, no reader is going to be hooked on your story.
One way to create tension is to use "tense" words. Provost suggests going through your manuscript and looking for places you can create tension by using words of delay, words that imply fear, words of danger, and words of urgency.
Below is an example of a paragraph I wrote which had some suspense. Then, I rewrote it with a few word changes as Provost suggests. The word choice definitely improved the tension.
Original:"She stopped at the sight of the broken gate, its rusty hinges damaged. He had come this way. Had he found the vault? She hurried past the broken gate and moved into the shadows. It was important she reach the vault before he learned the truth about her."
Revision:"She froze at the sight of the broken gate, its rusty hinges ripped loose. He had come this way. Had he found the vault? She rushed past the broken gate and stole into the shadows. It was critical she reach the vault before he discovered the truth about her."
Provost suggests going through your manuscript and looking for places where word choice can increase the tension in your story