"+" A leading plus sign indicates that this word must be present in each row that is returned.
"-" A leading minus sign indicates that this word must not be present in any of the rows that are returned. The "-" operator acts only to exclude rows that are otherwise matched by other search terms. Thus, a boolean-mode search that contains only terms preceded by "-" returns an empty result. It does not return "all rows except those containing any of the excluded terms".
(no operator) By default (when neither "+" nor "-" is specified) the word is optional, but the rows that contain it are rated higher.