What exactly is a motion tween? First, let's imagine that you place a box on the table. Now, imagine, that after 10 seconds, the box suddenly disappears and magically appears again a few inches to the right of where it had been originally. Pretty strange isn't it?
Well, if you could rewind your past minute and watch it again, what will you see? You will see the box suddenly disappear and appear again in a different position with no cohesion or "animation" in the 10 seconds that elapsed. Here is where motion tweening comes in. With motion tweening, instead of the square disappearing and reappearing, you will instead see the square slowly moving itself to its new position - no disappearing acts at all! A motion tween, simply put, fills in the missing sequences between a starting position and the ending positions to create a full animation.
This tutorial will explain how to create a motion tween in Flash MX 2004, and hopefully you will understand more about motion tweens. Your animation, toward the end, will look a tad bit similar to the following animation: