A subject heading is a term attached to a database record to describe a particular concept in that paper, regardless of the words or phrases used by the author(s). For instance, a paper discussing "car crashes" and a paper discussing "automobile accidents" would have the subject heading "Accidents, Traffic" assigned to them in PubMed.
Searching for subject headings can be used as an alternative (or complement) to keyword searching. It can also improve both the accuracy and the comprehensiveness of your database searching.
See the links below for descriptions of how to use Medical Subject Headings (MeSH) when searching PubMed: