With Boolean operators, you state how the search terms should be combined. The most commonly used Boolean operators in databases are AND and OR.
AND means that all the search terms or building blocks you have combined need to be appear in the found documents: caries AND fluoride retrieves documents which contain both caries and fluoride. The more terms you add with AND in between, the narrower your search will become. If you built your search with search blocks you should use AND between the search blocks.
OR means that at least one of the search terms needs to be found in the retrieved documents . You will get more hits for every word that you add combined with OR. Synonyms should be combined with an OR in between, such as canine OR cuspid.
A way to search for words beginning with the word stem followed by an asterisk (*). For instance prevent* will give you hits for preventive, prevention, prevents etc.Truncation broadens your search.
In this video you can learn how to use boolean operators, truncation and phrase searches in PubMed.
If you're conducting a literature review as part of your essay or thesis you need to create broad searches to find as much research on your topic as possible. This can be done using the building blocks strategy. In this video from John Hopkins University you can learn how to do it in PubMed.