The 19th century mathematician, George Boole, is credited with formulating the algebraic logic which was foundational in developing modern computing. It is this Boolean Logic which allows you to construct advanced search queries by combining keywords using AND, OR, and NOT to establish relationships between concepts.
Narrows a search by combining keywords to retrieve results which contain both/all concepts.
This is the most restrictive- the more keywords you seek to combine in one search, the fewer results you will get.
Example search: learning AND memory
Search results = 160,973
learning AND memory AND language
Search results = 20,428
Broadens a search by retrieving results which may contain either or both keywords.
This is the most expansive - usually, OR would be used to combine synonyms or closely related concepts.
Example search: learning OR memory
Search results = 4,475,410
Excludes unwanted terms from the search.
This decreases results retrieved by eliminating the keyword after the NOT operator.
Example search: learning NOT memory
Search results = 3,152,059
memory NOT learning
Search results = 1,164,031