Well, it's about the crayons having enough of the status quo. For example, the red crayon might be tired of always being used to draw hearts and firetrucks. They all decide to quit, which makes the kid realize how important each crayon is and how they should be treated better. It's really a story that can teach kids about respect and individuality among things they often take for granted, like crayons.
The main idea is that the crayons are tired of being used in a certain way or being ignored, so they 'quit' and write letters to express their feelings. It's a creative and fun way to show kids different personalities and needs, like how each crayon has its own unique situation and complaint.