Grist is grain that is ready to be ground, or has been ground, into a coarse powder or meal for making flour, bread, or animal feed. In old mills, farmers would bring their harvested wheat, corn, or other grains to be crushed between heavy millstones, and the resulting ground grain was called grist.

The word appears most often in the phrase grist for the mill, which means material or experiences that can be used for a particular purpose. If you're writing a story about your summer vacation, the funny moments, the boring parts, and the highlights all become grist for the mill of your imagination. A scientist studying weather patterns sees every storm as grist for the mill of research.

The expression comes from the practical idea that millers could grind any grain brought to them: all of it was useful, all of it was grist. Today we use it to mean that any experience or information, even difficult or unpleasant ones, can be turned into something valuable. When your teacher says that your mistakes are grist for the mill of learning, she means they're useful material for becoming wiser and more capable.