The MeSH database is located under "More Resources" on the initial search screen.  The database provides the capability to select Medical Subject Headings for searching and to limit the term to a major concept [MAJR].  Subheadings can be selected to refine the search.  Subheadings group together those citations on a particular aspect of a subject.  When a heading is selected, click on Add to search builder.  If desired add more headings combined with AND or OR.  When done, click on Search PubMed.