Metadata, simply put, is data about other data.
Imagine an invoice for a purchase your company has made. Think of the line items in the invoice as the data. This tells you what particular item was purchased, the quantity, the unit cost, etc. The metadata will be the stub, or the header portion, of the invoice. It describes what the line items refer to, who was involved in the transaction and when it happened.
Examples of metadata are file size, date created, who created it, and description of contents. They give you information on what to expect before accessing the data itself.