Use the pulldown menu to the left of the dialog box on the inititial search screen of PubMed to select MeSH. A link to the MeSH database is also located under "More Resources" on the initial search screen and near the bottom of the advanced 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.
MeSH is an acronym for Medical Subject Headings. It is the National Library of Medicine's (NLM) controlled vocabulary for indexing the articles found in PubMed.
MeSH terms are very helpful when searching for articles in PubMed. From the PubMed homepage, use the drop-down menu at the top left to search the MeSH database. Once you locate your term on the MeSH hierarchy, check the box next to it and then use the drop down menu above the results to "Send To the Search Box with AND." You can then do a second search for another MeSH term and combine these terms in the search box.
Here is a link to a tutorial on searching PubMed using the MeSH database.
Another tutorial on combining search terms using MeSH
And a tutorial on applying subheadings and other features of the MeSH database