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 input handlers
      as meta fields in the HTML header section, or
      extracted from file extended attributes, 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 handlers, 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.
The sequence of events for field processing is as follows:
During indexing, 
          recollindex scans all meta
          fields in HTML documents (most document types are transformed
          into HTML at some point). It compares the name for each element
          to the configuration defining what should be done with fields
          (the fields file)
If the name for the meta
          element matches one for a field that should be indexed, the
          contents are processed and the terms are entered into the index
          with the prefix defined in the fields
          file.
If the name for the meta element
          matches one for a field that should be stored, the content of the
          element is stored with the document data record, from which it
          can be extracted and displayed at query time.
At query time, if a field search is performed, the index prefix is computed and the match is only performed against appropriately prefixed terms in the index.
At query time, the field can be displayed inside the result list by using the appropriate directive in the definition of the result list paragraph format. All fields are displayed on the fields screen of the preview window (which you can reach through the right-click menu). This is independant of the fact that the search which produced the results used the field or not.
You can find more information in the 
      section about the
      fields file, or in comments inside the
      file.
You can also have a look at the example on the Wiki, detailing how one could add a page count field to pdf documents for displaying inside result lists.