Compostable refers to materials that can be broken down and decomposed into organic matter by microorganisms, such as bacteria and fungi, under the right conditions. Banana peels are compostable because they are made of organic matter and can be broken down into nutrients that enrich the soil. Shoes, blankets, and microwaves are not compostable as they are typically made of non-biodegradable materials like plastic, metal, or synthetic fibers, which do not break down easily in the environment.