Extending early is one of the three components of a "long swing"
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 cause of Extending Early might be trying to pull too much as pull conscience hitters want to get outside the ball and this causes early extension.
Some Drills that can help eliminate Extending Early are: