Keywords are words that carry content and meaning. The keywords in the research question "What is the feeding range of the blue whale in the Pacific Ocean?" are feeding range, blue whale and Pacific Ocean.
Think of words similar to your keywords in case a database doesn't use your original keywords. Synonyms for blue whale are baleen whale and Balaenoptera musculus.
A Boolean search is a search using the words AND, OR and NOT between the keywords. These words each have a special function when used in a database.
You can put a phrase in quotation marks to indicate that you want to find that exact phrase in the search results. For example a search with the keywords, quality of life may yield results that contain each of the three words in any order, whereas a search for, "quality of life" will yield results with only that exact phrase present.
You can avoid doing multiple searches for variations on word endings using the truncation symbol * (the asterisk) in most databases. Entering the keyword "blue whale*" will look for both blue whale and blue whales.
If you want a literature review, add "AND review" to your keywords. To find a research study, add "AND study" to your keywords.
Always go to the Advanced Search in a database to enter your Boolean searches because it gives you multiple boxes with the Boolean operators between them. If you are using a search with multiple search strings, enter OR within the search boxes and AND between the search boxes, e.g., [blue whale OR Balaenoptera musculus] AND [feeding range OR feeding grounds] AND [Pacific Ocean].