Tweening comes from the words "in between".

With Tweening you can go from one keyframe to another and specify changes in the animation and let the Flash program create the frames in between.

Step 1
Create a small circle to the left in the Stage area. Do this by selecting the circle tool from the left toolbar. Draw the circle in the Stage area.

Step 2
Select the Arrow tool from the left toolbar. Double-click on the circle to select it.

Step 3
Now we have to convert the circle to a symbol. When the circle is converted to a symbol we can create instances of the circle. From the top menu choose Modify > Convert to Symbol. Name the symbol "Ball", and select OK.

Step 4
Go to Frame 10 in the Timeline. Do this by clicking the gray field below 10. Then right click in this field. Choose Insert Keyframe. Keyframes appear as circles in a frame. This operation duplicates the image. 

Note: A keyframe specifies changes in an animation. You create keyframes at important points in the Timeline and let Flash create the frames in between.

Step 5
Select the circle and move it to the right a couple of inches.

Step 6
Click on the Timeline any place between Frame 1 and Frame 10. Then right click and choose Create Motion Tween.

Step 7
Choose Control > Test Movie from the top menu to test your Flash movie.