Okay, so have you ever played with blocks before? Proto-cubism is kind of like playing with blocks, but on a canvas.
It started a long time ago when artists began to experiment with the way they painted things. Instead of painting things exactly as they saw them in real life, they started using simple shapes like squares, circles, and triangles to create their art.
This style of art is called proto-cubism because it was the first step toward a style called cubism, which is all about using different shapes to create a picture. Just like how you can use different blocks to make a tower, these artists were using different shapes to make a painting.
Sometimes the artists would use these shapes to make something look like it was coming toward you, and sometimes they would use them to make it look like it was going away from you. They used different colors and shadows to make it look even more 3D than a real object!
Proto-cubism was a really cool way for artists to show off their creativity and experiment with a new way of painting.