Very often our searches are not confined to one topic. We search for articles on one or more of the following:
Boolean AND is used when we definitely want two or more of our search terms appearing in our search results
Every query has more than one term. These can be combined using the terms AND, OR and NOT. These simple words have a slightly complex name - "Boolean Operators"
If you wish to do a very thorough literature search in some contexts (eg if you are writing an exhaustive literature review), then it is good to supplement a NOT search with an AND search. To explain clearly - if you do a search:
But if there are articles that cover both terms, then when you type NOT lactation in the search box, you are eliminating such articles. So you lose information about pregnancy that may be valuable
So, it is good to then run a search - Pregnancy AND Lactation and hand pick articles that may have useful information about pregnancy