"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.
post traumatic stress disorder vs "post traumatic stress disorder"
premature infant vs. "premature infant"
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.
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.