Onion skinning is a special way of making pictures that move like a movie. It helps you to see how things move over time. It's like using many pieces of paper with different drawings on them and flipping through them really fast to see how they move.
Imagine you want to draw a picture of a bunny hopping. You would draw the first picture of the bunny with all its legs on the ground. Then, you would draw a second picture with one of its legs lifted up. You keep drawing more pictures, with each one having the bunny's legs in a slightly different position. When you put all of the pictures together, you can see the bunny hopping!
Onion skinning helps you to make sure that you're drawing the bunny in the right way. Instead of just drawing each picture on a separate piece of paper, you can use a special program that shows you the picture you just drew and the picture before it at the same time. This way, you can see how much you need to move the bunny's legs to make it look like it's hopping smoothly from one picture to the next.
Onion skinning is a great way to create animations, and lots of movies and cartoons use this technique to make the pictures come to life!