Major topics (Medline / MeSH)
Asterisks on MeSH headings and subheadings (e.g., Wound Healing/radiation effects*) designate that they are the major topics of the article, usually obtained from the title and/or statement of purpose
Non-major (non-asterisked) headings and subheadings are usually additional topics substantively discussed within the article, terms added to qualify a major topic (…).
The only indexed MEDLINE citations without an asterisked heading are some biographies in which the subject’s name may be considered the only major point. (…)
In resources with controlled vocabulary, you can map search terms to subject headings
Source: OVID help. Available form http://www.ovid.com/site/help/documentation/ospb/en/advanced.htm#map