Extending the arms is important in hitting but we want to extend (straighten the arms) through the ball in a forward direction (toward the pitcher). When a hitter extends too early it means she is pushing her hands more toward the plate (east/west) instead of toward the pitcher (North).
The hitter in the photo to the right is a pretty good example of a hitter that has not extended her arms yet and since her hands are in front of her (north) it looks like she will be extending forward and later instead of to the side and early.
You may have heard of "leading with the hands", "staying inside the ball"? These are what we want to do with our hands which is the opposite of extending early.
The best way to fix this is to work hard on a batting tee with it being close to you and out in front and work on hitting ball up the middle. There are a lot of other drills that are also effective in working on staying inside the ball and not extending early.