Reading is the ability to judge where the ball will go prior to the hit. If you as the defensive player have not narrowed down the possibilities prior to contact you decrease your opportunities for success. Traditional defensive systems place us in the areas where the opponents are most likely to hit the ball. However to reach many balls you must move rapidly since the area you must cover is large relative to the amount of time the ball is in the air. The best thought out defense is useless if the individual players are not able to anticipate the path of the ball and move from their base positions toward it prior to the hit.
Anticipation comes with experience.
Now turn your attention on the setter , watching the set ball until you determine the set direction and intended hitter. Next, watch only the hitter , judging. . . Off the net sets usually are hit more cross court. . . Conversely, if the ball is closer to the center of the net than the spiker's arm, either because the set is way inside or because the spiker is late in getting to the ball the hit will be cross court. If the spiker runs under the ball the hit will be deep or off-speed. . . .