What are MeSH?
Medical Subject Headings (MeSH) are the NLM's standardized terms used to describe the topics covered in MEDLINE. Articles about a topic are tagged with the appropriate subject headings, if available, whether or not the exact words of that subject heading are used in the article. Articles will be tagged with a subject heading even if the article uses some synonyms, spelling variations (e.g., British spelling), or different word endings (e.g., plural). Keep in mind, mapping to subject headings is not perfect, so employing other search techniques is also useful.
Automatic term mapping may pick up some MeSH for your search terms, viewable in the search details. You can also use the MeSH database to discover MeSH and get more context and information about them. See the MeSH in MEDLINE/PubMed tutorial for more information.