Imagine you have a sheet of paper and some markers or pens. You start drawing things on the paper, like a squiggly line, a circle or a square. You keep on drawing different shapes and patterns until your whole paper is full of unique drawings.
Now, imagine that instead of using paper, you have a long strip of clear plastic. You draw your shapes and patterns on this plastic strip using different colored markers or pens. Once you have drawn everything you want, you roll up the plastic strip very tight like a scroll.
Finally, you play the scroll in front of a bright light, like a projector or a flashlight. As the light passes through the transparent plastic, the shapes and patterns you drew start to move on the wall or screen like a moving picture. This is called drawn-on-film animation.
This way of creating animation was very popular before the use of computers in animation. It was used in films like "Begone Dull Care" (1949) by Norman McLaren and "Circles" (1933) by Oskar Fischinger. Today, it is still used by some artists and filmmakers who want to create a unique, handmade look to their animations.