When you do a search in PubMed, PubMed maps your search terms to the MeSH terms that are used to index articles. The search includes the keywords and the MeSH term(s).
Keyword searching is when you enter the search terms, or keywords, that most accurately describes your topic. This search strategy will find and retrieve the terms you entered.
One way to improve your search results is to use a controlled vocabulary. A controlled vocabulary is defined (controlled) list of terms that are assigned to articles in the database. If you can determine what controlled vocabulary terms are used for your topic you can search with the controlled vocabulary terms and find all the articles on your topic. PubMed uses MeSH, the Medical Subject Heading controlled vocabulary.
PubMed maps your keyword search to the appropriate MeSH, or controlled vocabulary terms, for you.
You can search the MeSH database to find the most appropriate search term, learn more about the MeSH term, see where the term is in the heirarchy, and build complex searches.