Nowadays we're mostly interested in metadata in the context of computer systems, but that's because computers are where you put data these days. The concept relates more to the data, and not so much to the machine. It's about how we can extract key elements of information on a consistent and orderly basis. The US Library of Congress maintains one of the world's great libraries. They've published their Modes of Cataloging Employed in the Cataloging Directorate online.
Subject access is through subject headings and classification.
Classification groups works by topic and is combined with a system of numbering individual items uniquely for purposes of physical access and inventory control. You may choose to do more with your metadata, but these tried and true cataloguing goals are a good place to start when you contemplate creating metadata. Resist the temptation to try to recreate your document - just provide information about your document from several points of view. A Unique Identifier for Each Item. It's worth reading if you want to understand the ongoing debate about identifying Web content and why this seemingly simple issue is getting complicated.