Use Boolean operators to combines terms when you search with more than one term.
When you search similar terms, you want to use OR. OR will get results that use at least one of the terms.
When you search different terms, you want to use AND. AND will get results that use all of the terms.
When you don't want results with a term, you want to use NOT. NOT will remove results that use that term.
You can use more than one operator.
Say you want information on heart attack recovery. The clinical term for heart attack is myocardial infarction, so you want to use that as well. Since they are similar terms, you combine heart attack and myocardial infarction with an OR. Recovery is different, and we want results that deal with both ideas. So now we can search for (Heart attack OR myocardial infarction) AND recovery.
When combining different types of operators, you need to use parentheses around groupings. "heart attack OR myocardial infarction AND recovery" is ambiguous.
(Heart attack OR myocardial infarction) AND recovery
gets different results than heart attack OR (myocardial infarction AND recovery)