Paint-on-glass animation is a type of animation where an artist paints directly onto a sheet of glass. Imagine having a clear piece of glass in front of you, like a window, and painting a picture on it with special paint that dries quickly.
Now imagine doing this over and over again, painting a slightly different picture each time, and then taking a picture of each painted layer in order. When you put all the pictures together, you get a moving image, just like a cartoon or a movie!
The glass is usually held up vertically so that the artist can paint on it from behind, using a special light to illuminate it. This creates a really cool effect where the finished animation looks like it's moving on a three-dimensional plane, instead of just being flat like other types of animation.