Fields are named pieces of information in or about documents, like title, author, abstract.
The field values for documents can appear in several ways during indexing: either output by filters as meta fields in the HTML header section, or added as attributes of the Doc object when using the API, or again synthetized internally by Recoll.
The Recoll query language allows searching for text in a specific field.
Recoll defines a number of default fields. Additional ones can be output by filters, and described in the fields configuration file.
Fields can be:
indexed, meaning that their terms are separately stored in inverted lists (with a specific prefix), and that a field-specific search is possible.
stored, meaning that their value is recorded in the index data record for the document, and can be returned and displayed with search results.
A field can be either or both indexed and stored. This and other aspects of fields handling is defined inside the fields configuration file.
You can find more information in the section about the fields file, or in comments inside the file.