Boolean operators (and, or, not) are pre-set established symbols used in online searching to specify and control the relationship between search terms. For example: AND requires all specified terms to be somewhere in the bibliographic record. It narrows your search by telling the search interface to find records that contain ALL of your search terms, e.g. A and B and C.
OR requires any or all of the specified terms to be somewhere in the bibliographic record. It broadens your search by telling the search interface to find records that contain ANY of your search terms, e.g. A or B or C.
NOT (sometimes phrased as AND NOT or BUT NOT) requires that the terms following NOT be excluded from the bibliographic record. It tells the search interface to exclude any record containing the search term following the word NOT. Records containing that term will not be retrieved, e.g. A not B. Because it affects everything which follows the NOT, the NOT phrase should be the last part of the entire boolean search statement.