Connectors (Boolean operators) are used in keyword searches to link together two or more terms. The most commonly-used connectors are: AND and OR.
AND narrows a search; you get fewer records, because both concepts must be present in the records found--here the area in color. Use AND to combine different concepts in one search.
OR broadens a search; you get more records because OR includes each of the concepts separately, as well as both concepts when found together. OR is often used to link together related concepts.