In short, metadata is data about data. The following brief description explains it well. It is from ESRI ARC/INFO help text.
Spatial metadata is data that describes the content, quality, condition, use limitations, and other characteristics of a spatial dataset. Metadata also documents bibliographic information about a geo_dataset, such as who collected the data, when it was collected, how it was collected, preprocessed, and converted, its resolution, who holds the data now, and so on.
Metadata is also referred to as additional information that is needed for a spatial dataset to be useful. Such information facilitates understanding of the data and its content between the provider and the user. It helps users to ensure that a dataset meets their needs and that they use the dataset appropriately.
Clearly, on-line spatial metadata, including a brief sample, is the most effective means to tell you if a dataset matches your project’s needs. It allows you to evaluate the benefits and shortfalls involved prior to purchasing and using a dataset.
Read our metadata.