"Exact phrase" searching involves putting two or more words inside quotation marks.
Putting words inside quotation marks turn them into a single word.
"Black cat" looks for results with these 2 words as a phrase.
Without the quotation marks, the tool will look for the words black and cat anywhere in the result.
Examples:
post traumatic stress disorder vs "post traumatic stress disorder"
premature infant vs. "premature infant"
vs. 
vs. 
Library search tools use a "controlled vocabulary" to tag each result. These tags are called subjects, subject headings, or indexing terms.
Subjects #tag the source and help you find others like it.
Often the subject terms are not words that you might expect.
Examples:
Instead of Dieting use REDUCING diets
Instead of High blood pressure use HYPERTENSION
Most search tools provide a list of their subject terms or indexing terms.
Sometimes this list is called a Thesaurus.
As you enter your search terms, the tool will give suggestions.
Suggestions are based on frequent searches and are often very effective to use.
They might lead you into a direction you had not considered.
