Keywords are the main concepts of your search topic put into single words or short phrases.
Before you begin any search, brainstorm a list of keywords that describe your topic. Identify the main concepts, use a thesaurus, dictionary, or reference source to figure out synonyms, more general and more specific terms related to your main concepts.
Consider synonyms: teenagers, adolescents
Consider spelling and word variations: ageing, aging
Consider broader or narrower terms: parents vs. mothers
This makes sure you find as much of the relevant information on your topic as possible.
Depending on what kind of information you are looking for you may need to use different combinations of keywords, or define phrases with quotation marks when you search databases.
AND narrows your search to results that contain both keywords:
teenagers AND depression
"assistive robots" AND healthcare
OR expands your search results to include items that contain either keyword:
dreams OR memories
NOT narrows your search to include the first keyword but not the second:
depression NOT phobias
How many keywords should you use when searching?
A good rule of thumb is to combine 2-3 of your keywords when performing a search.
Any more, and you may get very limited (or no) results. Any fewer and you may get too many results to easily sort through.