What does it mean for something to be compostable?

An item that is compostable will "decompose" over a short duration of time, back into natural materials such as compost. This is in contrast to materials like plastic or aluminum that require thousands or even millions of years to degrade and transform into something found naturally. The benefit of compostable materials is that they do not generate any trash to take up space in landfills, and use more naturally-derived inputs in production processes (in contrast to fossil fuels).