MeSH (Medical Subject Headings) is the National Library of Medicine's controlled vocabulary thesaurus, used for indexing articles for the MEDLINE®/PubMED® database.
Using MeSH allows you to find articles that are ABOUT that particular term, not just mentioning it. Also, there are many, many synonyms, or different terms that may be used in medicine (for example: stroke, CVA, apoplexy, brain attack, cerebral accident, cerebrovascular accident); using a MeSH term pulls in all articles on that term, no matter what synonym was used by the author.
If you can search using MeSH entry terms instead of keyword searching you can focus your search and find more relevant citations.