A field is a metadata type that contains a single piece of information about an entry. You might create an Author field that will contain the name of the document's author. You can apply fields to a document in groups, as part of a template, or independently. A document can only contain one template but can contain additional independent fields.
Fields are often used as a more efficient way to search for documents than full-text searches. When creating fields you should consider what types of information your users will be regularly searching for.