MeSH is the National Library of Medicine's controlled vocabulary thesaurus.
Each bibliographic reference is associated with a set of MeSH terms that are assigned to describe the content of an article.
This type of citation searching looks backwards in time. It starts with an article, often a fairly recent one, and looks back at the articles in the reference list. It is sometimes called the ascendancy approach (because it searches backward from an ascendant). It starts with a current article, and searches for articles that were written BEFORE the current article. These articles were used by the authors when conducting their research on the current article.