Tweening is like drawing a picture book on a computer! When you draw a picture, you can change it a little bit at a time to make it look like it's moving or changing. This process is called tweening.
Let's say you draw a stick figure on your computer. You can move the stick figure a little bit to the right, then save the picture. Next, you can move the stick figure a little bit to the right again and save it as a new picture. By doing this many times, you can create a series of pictures that show the stick figure moving across the screen.
The computer can then take each of these pictures you made and put them together into a video that shows the stick figure moving smoothly from one side of the screen to the other. This is called animating, and it's possible because of tweening!
Tweening helps the computer figure out how to smoothly move objects in the pictures you draw. Instead of you having to draw every single frame of the animation, you just draw a few key frames and let the computer figure out how to smoothly move between them.
So next time you watch a cool cartoon or movie where the characters are moving smoothly, remember that it's because of tweening!