It is how quickly or slowly your story moves. You can control the flow of the story by actions, sentence structure, and more. A quick, paced thriller will be written differently than a slow, moving family saga. Word choice, how quickly events transpire, and even dialogue can dictate how your reader feels. Example: consider a short, punchy sentence of a man on the run, and then the conversation of a man sitting and reminiscing with his grandmother on the front porch of their southern spread. When looking at these, you can see how long, flowing sentences would slow the reader down. These scenes are bathed in description, and are meant to move slower. You'll often find these "breather" scenes after a fast, paced action scene. That way the faster scenes has more impact.