I don't understand what this explain statement is saying. Which field needs an index?.

The first line to me is confusing because ref is null. Here's the query I'm using:

SELECT pp.property_id AS 'good_prop_id',
       pr.site_number AS 'pr.site_number',
       CONCAT(pr.site_street_name, ' ', pr.site_street_type) AS 'pr.partial_addr',
       pr.county
  FROM realval_newdb.preforeclosures AS pr
 INNER JOIN realval_newdb.properties_preforeclosures AS pp USE INDEX (mee_id)
  ON (pr.mee_id = pp.mee_id) 
 INNER JOIN listings_copy AS lc
  ON (pr.site_number = lc.site_number) AND 
  (lc.site_street_name = CONCAT(pr.site_street_name, ' ', pr.site_street_type))
WHERE lc.site_county = pr.county
LIMIT 1;

Can anyone help me optimize this query?